Boot with an OS install disk in drive, hold down the C key to force a boot from the disk. Find the utility Disk Utility and look to see if your hard disk appears. If it does, select it an Repair the disk. Repair permissions too if that is separate. See if this helps you reboot. If you ever get this hard disk to reboot, or mount on the desktop when booting from the CD it would be a good time to backup your important files. You might be close to a hard disk failure.
