First, try another cord. Also jiggle the cord and push on the connectors to attempt a more positive connection. See if that brings the drive to life on the system.
Try plugging the drive intro another USB port, preferably one of the ports connected directly to the motherboard. The most common problem with these portable drives is poor connections and/or poor power supply through the USB port or cable.
You can pull the drive out of the case and then connect it to a PC (desktop) using standard power and SATA connectors. This will test to see if the drive is good and if there is a problem with the interface in the case. How To Open Seagate Free-Agent Go Drive 2.5 in - YouTube