First try a safe boot, be paitent it will take longer than usual http://support.apple.com/kb/HT1564
If that fails insert your install disc and restart holding the C key until the Apple logo and spinning gear appear. Select your language and ignore the installer on the next screen, go to the menu bar and select Utilities-Disk Utility. Repair your hard drive, quit Disk Utility, quit the installer and see if your computer will start.
Still won't start? Restart again with the install disc and reinstall Snow Leopard. Run software update a few times to get everything up to date.