First you should really make sure it's the harddrive itself that's not working.

Does the harddrive have any light, and if so, does it blink when you connect the drive?

Does the harddrive make any sounds, if so, does it sound as it normally does or do you hear an unfamiliar "clicking"? (Clicking usually means the drive is dying)

Have you tried going into BIOS at startup and make sure that the USB (or whatever port it's connected to) is activated? Don't know why it would deactivate but always good to check.

Also, have you tried all ports in the computer, both front and back if you have any?

Have you tried going into the Windows device manager and checked if the drive is listed there?

If the drive is listed in "disk drives", remove it from there and reconnect it and Windows might find it again.

Also check disk management.