More than welcome , very useful site to be a member of
Try this , seeing as the drive worked previously :
Turn your computer off, then unplug the USB cable and then the power to the drive. Restart you computer, plug the power into the hard drive, wait 10 seconds and then plug the USB
into the computer. This clears the hard drive buffer. See if the drive shows up in disk management. Before formating, turn off your antivirus.
Try using a different USB port also, forcing windows to recognise the hd . Also make sure that the jumpers are set correctly ( dependent on make of hd )
Did the external HD make any strange noises when you had it working before ? Whirring or grinding noises are a surefire sign the internal motor on the HD is on it's way to HD heaven .
Also check if the external drive casing requires any specific drivers - most are plug and play but some external drive enclosures require specific drivers .