First, see if recovering the partition and format info will solve the problem. Free utilities such as the one from EASEUS ( http://www.easeus.com/datarecovery [...] ftware.htm ) will see if they can recover the partition information and make your data available again.
If that is not the source of the problem, there are utilities to scan the drive sector-by-sector and try to reconstruct all of the files, or at least those are not corrupted. You can search the forums for "drive recovery" and find some. These are usually free to scan your drive but pay to recover more than one sample file.
And the most important advice: In the future, do backups. To a drive that is only attached to your system while you are doing the backup, and then stored separately.