Try Repair the disk
NOTE: refer to article "Resolve startup issues and perform disk maintenance with Disk Utility and fsck" found here - http://support.apple.com/kb/TS1417 - for the actual apple article on this step. --
Also see - http://support.apple.com/kb/HT1452.
- with computer on, insert the computers "install disk 1" or "Mac OS X Install Disk" into the optical drive.
- shut down computer
- restart computer holding down the letter 'C' on the keyboard. Keep holding down the 'C' key, while the computer powers on, and continue to hold the key down, until you see a screen appear, asking for you to select a language. This may take a minute or so to appear. When this screen appears, release the 'C' key
- select english, then the continue arrow in the lower right corner
- the next screen will display "Welcome to the Installer". On this screen, we want to look into the top menu bar (beside the apple in the top left corner) and select 'Utilities' , then 'Disk Utility' ---
- When prompted to select a disk volume or image, select the 'Macintosh HD' in the left column
- ensure the the "first aid" tab is highlighted blue in the middle of the screen
- click the button at the bottom that says 'REPAIR DISK' .......NOT "Repair disk permissions"
- this process will take a few minutes to run. It will indicate it's done, by scrolling to the bottom of the "details area" displaying "The Macintosh HD appears OK"
If you get any other message OTHER than "The Macintosh HD appears OK", like "the Macintosh HD was repaired successfully" run the "Disk Repair" again, until it displays "The Macintosh HD appears OK".
Once done, beside the apple, click on the words "Mac OS X Installer", and select quit / restart. On the keyboard, hold down the eject key (top right corner, grey triangle pointing up, with a line underneath) until the disk ejects from the drive. Just let the computer start up normally.
NOTE - if you don't have or can't locate the install disks, you can follow the section 'Use fsck' of the same above article to boot into single user mode, and run the /sbin/fsck -fy command. This essentially runs the same test, that is on the install disk, just without the disk present---
If that doesn't work, you can try re-install the OS via an archive and install.
To do an archive and install:
- insert install disk 1 into the computers optical drive
- restart computer holding down C
- Select english as your language, and then continue
- on the next screen simply select continue - select the Mac HD as the destination, then click install