To help diagnose your problem, use Disk Utility:
1. Insert the Mac OS X Install disc that came with your computer, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Mac OS X volume.
6. Click Repair. Disk Utility checks and repairs the disk.
If you can't boot from your install disc, or in single user mode ( Start in Single-User-Mode(Hold the cmd()+S when you hear the tone and release when you see the Apple logo) this will give you a black screen with text (command-line) wait till it finishes booting up and you have a command prompt(localhost:/root#) then type in (/sbin/fsck -fy) this verifies and repairs your startup drive. If it finishes and reports the hard drive has been modified---repeat the command-line entry as above.
Do that until it report the hard drive is OK. then type in (reboot).
If that doesn't work either then the best thing is to take it in to Apple.