You need to have a software like Easy Recovery Professional, it can retrieve datas even if is formatted. All you need is to install the software to a windows xp running OS and slave the HDD you where you want recover datas.
Depends how the disk was formatted. If you used special formatting software that might have written "0"'s to all the tracks, fully erasing the disk.
If it was just an accidental normal file system format, I believe all that happens is that the table of pointers that say where everything is on the disk is erased and a blank one created. The data is still there unless you start overwriting it by creating any new files on the formatted drive. Usually you can get almost everything back although it can be a long drawn out process of recovery. Special software like Hans mentioned will sort that out.
This is what causes the big news stories every now and again where some NHS/bank IT guy just formatted the disk (or not in some cases) and then skipped the PC. Someone finds it and recovers all sorts of private information.
If you are so desperate that you want your file back no matter what it takes and the file is something reasonably dechiperable like a word document you can trawl through the disk using a low-level hex editor (there are loads available on the 'net) and piece it back together