There are a couple of possibilities here:
* The USB cable is bad - Try a different cable
* The USB port you are using is bad - Try a different port on the system
* The Drive needs more power than your port is able to offer - Try using a external powered USB hub or power supply for the drive
* Try a second Mac system to see if it can access the drive.
Lastly, see if Apple's ''Drive Utility'' is able to see the drive. If it does try repairing it.