It sounds like it may be a hard drive that has gone bad. I would try plugging it back in to the computer and right clicking on Computer (My Computer on WinXP) and selecting Manage. Then click on Disk Management. This should show you if the drive is truly readable. If it does show up here, then it could be that a sector went bad that is preventing it from showing up. This can be fixed with a format. If it still is not showing up here, then I would say that the drive has failed.
In either case, if the drive has information that you truly need, then I would suggest taking it to a hard drive recovery place. This service is very expensive though, so only do this if the information is extremely important.