Could be many things, we should start with this...
"Try Disk Utility
1. Insert the Mac OS X Install disc, 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 at top of the screen. (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. Select your Mac OS X volume.
5. Click Repair Disk, (not Repair Permissions). Disk Utility checks and repairs the disk."
Then try a Safe Boot, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, reboot when it completes.
(Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive & clear caches.)
If perchance you can't find your install Disc, at least try it from the Safe Boot part onward.
Gret news thanks!
At this point I think you should get Applejack...
After installing, reboot holding down CMD+s, (+s), then when the DOS like prompt shows, type in...
Then let it do all 6 of it's things.
At least it'll eliminate some questions if it doesn't fix it.
The 6 things it does are...
Correct any Disk problems.
Clear out Cache Files.
Repair/check several plist files.
Dump the VM files for a fresh start.
Trash old Log files.
First reboot will be slower, sometimes 2 or 3 restarts will be required for full benefit... my guess is files relying upon other files relying upon other files! :-)
Disconnect the USB cable from any Uninterruptible Power Supply so the system doesn't shut down in the middle of the process.