What version of Windows? You might want to try to unplug the device from your computer first. Then remove all the usb drivers from your computer, and then do a restart see what happens. If you can plug it into another computer. It may just be the enclosure.
Okay since you have tried it on another computer it seems to be a bad hdd or a bad enclosure. Always good to have an adapter like the Apricorn handy. Keep in mind taking the hdd out of the enclosure voids any warrantys; although, your data may be more important. If it is the hdd, start by running a scan with GetDataBack, if that doesn't work, you can send it off to drivesavers or so on. Their rates start at $500+ depend on how much data you have.