When it was working, did you used to disconnect it properly before you unplugged it from the computer? For example right clicking the drive and selecting 'remove safely' or 'eject'. If you haven't then it could have corrupted the hard drive, this especially happens when you are writing to the external hdd and do not properly disconnect them.
Other thing you can try is goto Control Panel -> Administrative Tools -> Computer Management and from the left hand panel expand Storage -> Disk Management and see if it come up as a valid partition on the right hand side.
Even if its corrupt you can use a data recovery tool to get data back. I recently used GetDataBack from
http://www.runtime.org/ to recover data from a corrupt memory card.
Ali