Start up from a system software CD or DVD, and repair your disk using Disk Utility:
Insert the Mac OS X Install or Restore disc.
Restart the computer, then hold the
C key during startup.
From the Apple menu, choose Disk Utility. Do this in the first screen of the Installer. Don't click Continue. If you click Continue in a Mac OS X Installer version earlier than 10.2, you must restart from CD again.
Click the
First Aid tab.
Click
Repair Disk.
After repairing the disk, try to start from the Mac OS X hard disk.