|I'm assuming you have NOT dropped the drive. |
An external hard drive MUST be plugged into a USB port (or two USB ports) that can supply 500ma of current, and that can recognize it otherwise.
If you DO have it plugged into such a port, if it isn't showing up in My Computer or Windows Explorer, or if it is but you can't access some or all of the files you know you had on it previously, the most common reason for that is you unplugged it while Windows was running WITHOUT clicking on the Safely Remove Hardware icon and choosing to STOP accessing the drive, and not doing that damaged data on the drive. Usually that problem can be fixed by using certain data recovery programs.
Troubleshooting USB device problems including for flash drives, external drives, memory cards.
See Response 1: