If you actually wrote a new file system to the partition then you are SOL. You would need a professional recovery service, and they would only be able to recover so much. But if all you did was change the partition table you could theoretically try to change it back to what the old scheme was. But you would need to have as much info as possible, like the starting block location and block size. You could also try to look for a tool that would look at various blocks and try to piece together which ones were part of the same file. But i think this might not work on drives that were too fragmented.