No uncommon for external USB (it is USB?) HDD's not to be recognised - normally due to unassigned drive letter (unless it's not formatted! - check using below also)
Run Disk Management (via e.g. Control Panel, then System and Security, then Administrative Tools, then Create and Format hard Disk partitions).
Hopefully your HDD appears (if not, unplug then plug in, wait....might have to repeat a couple of times). Recheck that it's formatted, right mouse click on disk and select Change Drive Letter and Path then, if no drive letter appears, assign one.