What happened? Did it show up before on the same PC? Were you transferring stuff when it stopped all of a sudden?
Do you get a clicking sound from the external disc when you plug it in?
Is it the Drive with the extra power or is it a USB powered drive?
Lot's of unknowns.... You need to tell us what happened and what all have you tried, have you tried putting another USB drive or device in the same USB port in which you have plugged the ExtHDD? Did it work?
Did you try plugging the ExtHDD into a different USB port? And what happened?
Did you try plugging the ExtHDD into another PC and see if it showed up there or not?