It may not be repairable. Test it with "SeaTools for Windows" then post the result.
The component that usually fails first on an external hard drive is the PCB inside the enclosure. Download SeaTools here: http://www.seagate.com/support/downloads/seatools/
You can try removing the drive from the external enclosure and mounting it internally to your pc or into another external enclosure as sometimes its the enclosures sata to usb interface that is having the problems.
Removing the drive would void any warranty on it. you''ve been warned.