Is it attacked by some virus recently?
As I know, the virus sometimes would hide your files on the drives and marks them as system file. And at the same, some related shortcuts would also be selected so that you cannot find your original data. So, in this case, the inner data may be still there.
Go run your anti-virus scan on your computer and try to find back your files.
Doesn’t work?
If it is not the virus problem, your drive may be corrupted. In order to get back your files and this hard drive, you could try some data recovery software online to help you.
If you are suspicious about data recovery stuff, you could firstly try some drive data recovery freeware:
http://www.piriform.com/recuva...