Try to assign a new letter to it:http://technet.microsoft.com/en-gb/library/cc757491%28v=ws.10%29.aspx
If this doesn’t make a sense, you should try a free hard drive recovery tool
to restore your inaccessible data at first. It will restore your data back just in the same way as it successfully restores the data of my friend completely from a RAW hard drive. You could give it a shot: http://raw-file-system.blogspot.com/2013/02/raw-drive-recovery.html
At last, you could also format it to see whether it could work normally.
In the future, you should keep at least two copies of everything important on separate drives.