What hardware or software changes have you done on the machine that caused the problem to start? Have you tried using System Restore and choose an earlier date (like a day or so before the problem developed)? Is the drive visible in "My Computer" and "Device Manager"? Is the drive flagged as having a problem in "Device Manager"?
Go here:
Code:
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00817052&lc=en&cc=us&dlc=en&product=3752864&os=2093&lang=en
and use the troubleshooting instructions to see if you can fix the problem.