blue screen

Have you made any changes recently? Replaced hardware, added new external hardware via cables? If so, unplug any external device.

Are you able to boot in safe mode? Hold the shift key down while booting. This boot will take longer than normal, due to disk maintenance that's done. If this works, try booting normally.

If that doesn't work, boot to an original OS X DVD or a retail OS X DVD, bring up Disk Utilities off the menu bar Utilities, and repair the hard drive and repair permissions. Try to reboot normally after that.

If that still doesn't get things going, reset the PMU and PRAM.

PRAM:

1. Shut down your Mac.
2. Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.

Next would be reinstalling the operating system.