You will need some kind of disk editor or sector editor. I had to do this almost 20 years ago with a DOS disk on a Novell 3.10 server (Lost a single hard disk head, person backing up was not doing it, correctly. Backing up was not my task, but I was willing to give the recovery a shot) You might also google computer forensics. The best thing would be use some sort of live CD. If your data was stored in a text file, it should be easy. If your data is compressed it is still do able, but a lot of work. If the data was encrypted, I am not sure you will be able to recover it even if you have the key.
In short, you are going to have to search the disk sector by sector for your data and put the file back together, piece by piece.