Dissapearing HD

Hey guys, I need some help pertaining to dissapearing drives.  My friend's 10GB HD decided to dissapear from his computer, he's running WinXP service pack 2... and that's really all I know of his computer.

Anyway, besides the fact that the HD might have died on him what else could be the problem? Is there some kind of tool that'll search for the drive or something? :\

Never dealt with this before... Any help would be greatly appreciated!

Had the exact same problem on my brother's machine, and two or three machines at work. It turned out in all these cases there was a loose wire in the power connector to the hard drive, which meant any vibration (footsteps, etc.) could jiggle the wire and cut the power to the drive. Pause the boot process (or go into the BIOS screen) and wiggle the power connector, and listen carefully to see if you hear the drive spin up or spin down.