Most external drives these days are USB connected.
USB device problems including for flash drives, external drives, memory cards.
See Response 1:
After checking out those things,
If the external enclosure is not detected by Windows, in at least Device Manager - USB Controllers as a mass storage device, then the hard drive inside of it can't be detected, and you must remove the hard drive and connect it internally to a computer, or install it in another external enclosure, in order to check it out, however I have found you cannot run hard drive manufacturer's diagnostics on a hard drive that's inside a USB connected external enclosure.
If the external enclosure IS detected AND the hard drive DOES show up in Device Manager under Disk drives and in Disk Management , but you can't access the drive in My Computer or Windows Explorer, if you're SURE you have it connected to a USB port that can supply 500ma of current, then you need to try one of the programs suggested to attempt to recover the data.