Unless it is a really large enclosure, it probably doesn't have a fan. What you thought was a fan is probably just the disk itself spinning. Although that doesn't really solve your problem, it does point that its not getting powered properly.
Something in the circuit board taking power from the wall/computer to the drive may be messed up. Is this a physically big drive that gets its own power from the wall? Or is it a small drive that just plugs into USB?
If its a big drive, you could open it up and put the drive in your computer (if your computer is a desktop). If its a small, you can still probably get the drive out, but it won't work without adapters in your desktop. If you have a laptop.. Then you have fewer options.
tl;dr - sounds like your enclosure's board might have died.