If the external HDD has a power connector, make sure its plugged in and connected. If it doesn't work in one USB port try different ports. If that doesn't work, try it in a different machine. If that doesn't work then the external HDD enclosure may have gone bad and/OR the HDD itself may have failed. If the HDD itself has not failed, then it can be removed and put into another external HDD enclosure (or into a desktop computer) to get it working again. If you are uncomfortable doing that yourself, any computer repair shop can help you out. But at least make sure its got power and try it in a different machine.