|Don't confuse the External enclosure with the actual hard drive. There are two components that make up the unit. The USB enclosure may show in Device manager as a USB device.|
Look in Disk Management to see if the hard drive shows there. If it does post how it is described there.
Data is written by magnetic impulses so you probably have either corrupted or totally wiped the drive.
If you want to attempt any data recovery then try one of the program linked below.
If all you want to do is make the drive usable then download the drive fitness and drive prep tools from the drive manufacturer's site and use them.
PC Inspector (freeware)
Zero assumption Recovery
The demo is limited
It will only recover 'up to' four folders per run
But you can make multiple runs
IMO the best, but not free