As most people who have a Mac already know Mac’s have many of the same issues as PCs. Here are some of my recommendations to solve Mac Boot issues.

• Run the hardware diagnostic that comes on the install DVD(s) that came with the Mac (note: these will not be available on a "generic" OS X disc bought separately from the Mac). Depending on what version of OS X your computer shipped with, put in either Mac OS X Install Disc 1 (through 10.5.4) or Applications Install Disc 2 (10.5.5 or later), then boot holding down the D key; seeKB #HT1509 for details.
• If you have AppleCare, that should have come with TechTool Deluxe, which you can use to run some additional hardware tests.
• If you don’t find any hardware problems, Check the integrity on the disk. Boot from the install DVD (hold C this time, so it’ll boot the installer rather than diags), select a language, and then run Disk Utility from the Utilities menu. Select your boot volume in the sidebar, then the First Aid tab, then the Repair Disk button near the bottom right. This will run an integrity check on the volume structure (i.e. it won’t check the contents of the files, just make sure the data structures that keep track of them are consistent) and repair any problems it finds. If it finds problems, I’d recommend running it repeatedly until it gives a clean bill of health.
• While you’re still on the First Aid tab, run Repair Disk Permissions near the bottom left. This sill set the permissions on the OS files back to what the install packages specified form them. Note that it only fixes the OS itself, not user files or even third-party software. Ideally, this should be run from within the booted OS, because there are some slightly version-specific tweaks, and the OS version on the DVD is probably different from what’s on the hard drive; but it should get things 99.9% correct, and if that makes the system bootable you can then re-run it properly from the HD to fix anything the DVD version didn’t get right.
• If it’s still not booting properly after that, try some alternate boot modes to get a better idea what’s going wrong. I’d try verbose mode first: hold Command–V while booting, and (if it gets far enough) it’ll go to a text interface where it prints information about what’s happening as it boots. This won’t fix anything, but may give an idea what’s going wrong.
• Next, try running the computer in Safe Mode by booting with the Shift key down (see KB #HT1564 and#HT1455) — this disables noncritical system extensions, third-party daemons, and a bunch of other possibly-problematic system components. You’ll be missing some functionality (IIRC sound and wireless networking both get disabled), but if you can successfully boot in this mode, you’ll know the problem is coming from one of the things that get disabled in Safe Mode; play with disabling & reenabling them to isolate the problem further.
• Finally, if that doesn’t solve it, try booting in single-user mode by holding Command–S while booting. This interrupts the boot process early on, and drops you into a very minimal full-screen command-line interface. If you know a lot about how OS X works, you can dig around in this mode to find/fix problems, but at this point I’d just consider it a test: can the OS boot far enough to get to single-user mode? BTW, to get out of single-user mode, the standard thing is to use the "exit" command to continue the boot process where it left off; since that’s failing for you, I’d instead use "shutdown -h now" to shut the computer off.

Hope these steps help you trouble shoot your Mac boot Issues!

I want to thank my dear friend Tamara for teaching me something new about Macs today.

A mac at one of the places I work at had it’s wireless mouse stop working (Wireless Bluetooth mouse to be specific). It turns out that the bluetooth controller on the machine wasn’t being detected. The only way it could be fixed was resetting the System Management Controller (SMC).

But wait, how could this happen! It’s a Mac right? Any who, she fixed it by turning off the Mac, unplugging it completely and then holding down the power button while being unplugged. She recalls holding the button down for around 5 to 10 seconds.

Plug the iMac back into the power source and turn it on. You might be surprised that somehow magically the bluetooth is back up and running on the machine!

I’ve included the info here for how and why from the apple KB

Note: It is possible that the SMC could encounter an issue that may cause unusual system behavior typically related to the symptoms described below. In some cases, resetting the SMC may be the only correct method to resolve the issue, however, an SMC reset should only be attempted after all other standard troubleshooting has been performed.

Before Resetting the SMC – Try each of the following steps in this order before you reset the SMC. Test the issue after completing each troubleshooting step to determine if the issue still occurs.

• Press Command + Option + Escape to force quit any application that is not responding.
• Put your Mac to sleep by choosing the Apple () menu from the upper-left menu bar and then choosing Sleep.
• Wake the computer after it has gone to sleep.
• Restart your Mac by by choosing the Apple () menu from the upper-left menu bar and then choosing Restart.
• Shut down your Mac by by choosing the Apple () menu from the upper-left menu bar and then choosing Shut Down.

If your Mac appears to be running normally but isn’t responding, force the computer to shut down by pressing and holding the power button for 10 seconds. Note: You will lose any unsaved work in any open applications.

For Mac portables experiencing issues related to the MagSafe power adapter and/or battery, try:

Unplug the MagSafe power adapter from the Mac and the wall power socket for several seconds.
Shutdown the Mac. Physically remove and re-insert the battery (if it’s removable) and startup the Mac.

If the issue is still not resolved after following the troubleshooting items above, an SMC reset may be necessary.

After performing normal troubleshooting, these symptoms may indicate that an SMC reset may be necessary:

Fans – The computer’s fans run at high speed although the computer is not experiencing heavy usage and is properly ventilated.

Lights – The keyboard backlight appears to behave incorrectly (on Mac computers that have this feature).
The Status Indicator Light (SIL) appears to behave incorrectly (on Mac computers that have an SIL).
Battery indicator lights, if present, appear to behave incorrectly (on portables that use non-removable batteries).
The display backlight doesn’t respond correctly to ambient light changes on Mac computers that have this feature.

Power
The computer doesn’t respond to the power button when pressed.
A portable Mac doesn’t appear to respond properly when you close or open the lid.
The computer sleeps or shuts down unexpectedly.
The battery does not appear to be charging properly.
The MagSafe power adaptor LED doesn’t appear to indicate the correct activity.

System Performance
The computer is running unusually slowly although it is not experiencing abnormally high CPU utilization.

Video
A computer that supports target display mode does not switch into or out of target display mode as expected.
A computer that supports target display mode switches into or out of target display mode at unexpected times.

Resetting the System Management Controller (SMC)
Resetting the SMC on Mac portables with a battery you can remove

Note: Learn about removing the battery on MacBook and MacBook Pro.
Shut down the computer.
Disconnect the MagSafe power adapter from the computer, if it’s connected.
Remove the battery.
Press and hold the power button for 5 seconds.
Release the power button.
Reconnect the battery and MagSafe power adapter.
Press the power button to turn on the computer.
Resetting the SMC on portables with a battery you should not remove on your own

Note: Portable computers that have a battery you should not remove on your own include MacBook Pro (Early 2009) and later, all models of MacBook Air, and MacBook (Late 2009).
Shut down the computer.
Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
Release all the keys and the power button at the same time.
Press the power button to turn on the computer.
Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.
Resetting the SMC for Mac Pro, Intel-based iMac, Intel-based Mac mini, or Intel-based Xserve

Shut down the computer.
Unplug the computer’s power cord.
Press and hold the power button for 5 seconds.
Release the power button.
Attach the computers power cable.
Press the power button to turn on the computer.
Additional Information
The System Management Controller (SMC) is responsible for many low-level functions on Intel-based Macs. These functions include:

Responding to presses of the power button
Responding to display lid opening and closing on portable Macs
Battery management
Thermal management
The SMS (Sudden Motion Sensor)
Ambient light sensing
Keyboard backlighting
Status Indicator Light (SIL) management
Battery status indicator lights
Selecting an external (instead of internal) video source for some iMac displays

Reset 10.5 Leopard & 10.6 Snow Leopard password

1. Power on or restart your Mac.
2. At the chime (or grey screen if your chime is turned off), hold down Command+S on your keyboard to enter single-user mode.
3. This step is optional, but it’s a good idea because it checks the consistency of the hard disk before moving on. At the prompt, type fsck -fy and press Enter/Return. Wait for the checks to complete before going to the next step.
4. Type mount -uw / and press Enter.
5. Type launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist and press Enter.
6. Type ls /Users and press Enter. This lists all of the usernames on the computer – helpful if you don’t know or remember what these are.
7. Type dscl . -passwd /Users/username password and replace “username” with one of the users displayed in the previous step. Replace “password” with a new password of your choice. Press Enter.
8. Type reboot and press Enter.

Reset 10.4 Tiger password

1. Power on or restart your Mac.
2. At the chime (or grey screen if your chime is turned off), hold down Command+S on your keyboard to enter single-user mode.
3. Type sh /etc/rc and press Enter/Return.
4. Type passwd username and replace “username” with the short name of the user account for which you’d like to reset the password.
5. Type your desired password and press Enter. It won’t show itself on the screen, so be careful what you type. You will most likely have to enter it again to confirm.
6. Type reboot and press Enter.

Tricking your Mac into creating a new user account

1. Power on or restart your Mac (should work for any Mac OS X system).
2. At the chime (or grey screen if your chime is turned off), hold down Command+S on your keyboard to enter single-user mode.
3. This step is optional, but it’s a good idea because it checks the consistency of the hard disk before moving on. At the prompt, type fsck -fy and press Enter/Return. Wait for the checks to complete before going to the next step.
4. Type mount -uw / and press Enter.
5. Type rm /var/db/.AppleSetupDone and press Enter.
6. Type shutdown -h now and press Enter.

The last way of resetting your Mac’s password requires some additional explanation. Instead of changing the passwords, this one fools the operating system into believing it has never been started before. This means when you restart the machine, it will take you through the entire setup & registration process all over again. Do not worry, all of your pre-existing data is still available to the hacker. Just continue with this process and do not choose the option to transfer or migrate information to the Mac.

Once you’re done, log in with the new admin account and go to System Preferences > Accounts. You will have to click on the lock icon in the lower-left corner before making changes here. You will see the original user account(s) on the left hand side. Convert the user to a standard user account (uncheck “Allow user to administer this computer”) and change the password. You should be able to log in to that account now and access all of your files and programs. If you want, you can log back in with your new admin account and re-check the “Allow user to administer this computer” option in System Preferences to grant admin privileges to the original user again.

That should cover it! As always, remember messing with single-user mode and Terminal is risky and may lead to trouble if you make mistakes. Considering you’re already locked out of the computer, though, you’ll probably be willing to take your chances at this point.

One last thing… if you have a Keychain Access password, you will probably need to reset that as well. You can do this by selecting “login” under Keychains on the left and choosing Delete from the Edit menu. You’ll lose everything in the Keychain, but you’ll now be able to start fresh and add new ones.