The drive may simply have failed. Run diskmgmt.msc and see if the drive is listed there. If it check its status and reinitialize it if you have too, just remember to reassign it a letter after you do this. All data will be lost.
If you continue to have trouble, create a Parted Magic boot CD (http://partedmagic.com/doku.php?id=start)
and plug the drive into a port on your computer. Restart the machine and boot from the CD. Once you are in, run a disk diagnostic. This will give you a good idea of the drive's health.