External Hard Drive (USB) Not Recognized on PC

Im having Hitachi 100GB External Hard Drive and for last 10 months it work fine. But a week ago one of my friend took it and send me it back and after that its totally dead, not recognize by my laptop and either on other machines. I dont have a contact of my friend cos he left his place and i dont know what he has done.

What sort of a problem it may be? Software? or Hardware?

Once i connect it green color light is fully lighting and not blinking as before and it giving me a error that USB device is not recognised.

How to get rid of this?
Please Help.

Take the hard drive out and put it into your computer.
Download the drive diagnostics from the drive manufacturer and run them on the drive.

Can you supply us with some more information about your external HDD, please? What is the exact model and make?

If you have warranty on this unit, I'd see about having them take care of it. Opening the unit will void your warranty. If you do not have warranty, then try out what DosFreak suggested.

Thanks for replies.
Yep i hv warrenty, But the shop which i purchased far away from my place.
So Lets try ....