Have you made any changes to your computer, around the time you noticed this problem? If so, what are they?
Have you tried plugging it into another computer, to retreive the files? If your drive works in another computer, then we have narrowed the problem down to your computer. If it doesn't work in another computer, then the problem is likely the drive.
You could try booing into
Safe Mode to see if your computer sees it......Restart>tap the F8 key>select "safe mode" If your computer sees it in Safe Mode, then likely there's a driver conflict with your system, or an application conflict.... If this is the case and you don't know what program is causing it, do a System Restore....
make sure to back up, in the future