Check the usb cable to see if it is correctly linked, try it on another computer, change the connecting cable, if all these do not work maybe you will need a data recovery software
this is a free version and you will be able to recover 1GB files. After the installation, firstly link your HD to the pc, secondly click deep scan. Wait till the scanning is over and then preview the files need to be recovered. And save them in another place.
If the software still cannot help you then you may have to send your HD for repair.