First, copy all the files off it to something else in case it is failing. Sounds more like the fault of the external enclosure than the drive.
Second, what machines have you tried it in? More than one? Is it possible that the USB 2.0 on you machine is dead? Can you test a 2.0 device in the port where it can't be read?
Please let us know what tests you have done - different ports, USB 2.0 ports on different machines, none? Then we can get a better idea.