1. Connect the drive to a different computer and attempt to access the drive. Swap the USB cable. Ensure that the power supply is good (test current from it with a multimeter) and that the connectors inside the external chassis are still connected to the drive. If it doesn't work...
2. Try data freeware recovery tools - TestDisk, Recuva, PhotoRec, Restoration, Undelete Plus. If they don't work...
3. Wrap the hard drive in foil, put it in a Ziplock bag, and put it in the freezer overnight. This may buy you enough usage to immediately copy all data off of the drive. If this doesn't work...
4. You will have to send the drive out for data recovery services. They will take the drive itself apart in a class 100 clean room, and migrate the platters to another set of working electronics and a freeform read head. They will read all data from the magnetic platters that hasn't been physically impacted, and will send you the resulting data on a hard drive or DVDs. This costs about 150,000 Ksh. Ontrack data recovery is a good company to use for this service.
I sincerely hope that you backup up your data.
Never buy ONE hard drive. Always buy at least two. Three is better, with one stored off-site.
Hill