To exclude some sort of software problem, try to boot the computer from a DVD disc (not a CD).
The easiest way to do this is downloading a live DVD Linux distro like Knoppix. Once you have the DVD, boot the computer from the disc. If the system loads with no problems, then the drive is not damaged, and we must search for the problem somewhere in the operative system or in the installed software.
Did you install a software recently, just before this problem arose?