Could mean anything, varies with the drive manufacturer.
If all the things the Capt and I suggested don't work, my first step would be to get (or borrow from someone) another external enclosure, and hook up the drive inside your dead enclosure to the new one. If it still doesn't work, then you know it's your drive, not your enclosure (thought it could be both). If you happen to have a spare drive lying around, you could try the reverse by putting the other drive into your dead enclosure.