Is it a network drive? Or is it directly pluged-in via usb?
Have u already tried to look for it in the disk manager? Some times, if you install a drive emulator of some kind or even a flash drive, it will use the same letter assinged to your external drive. All you have to do is go in the disk manager and change that letter.
Disk Manager can be accessed by right-clicking My Computer and the in Manage. In that window you'll find de disk manager. See if your drive shows up. If not, there may be a problem with your connection (cable, disk board). Try and see if at least it shows up on your BIOS. If all fails, I would try the suggestion mentioned by four4magicparts above.