It hasn't been written over with new data on the disc---When your operating system deletes a file, all it really does is mark the space on your hard drive that your file occupies as free space. It's still there, but your computer is now perfectly happy to write new data on top of it-at which point the file recovery process becomes a lot more difficult. If you formatted your computer and did it with a different file system type, the recovery is going to be a whole lot more difficult.
If you are using Windows, there are a lot of options available as free file recovery programs. Some of them are Recuva, Undelete Plus, PC Inspector File Recovery, and Restoration.
MAC users might have to spend a few bucks and get Data Rescue II.
If you are using Windows 7 or Windows Vista Ultimate/Business/Enterprise editions, then the shadow copy feature provided by them may come to your rescue. Shadow copy creates copies on a scheduled basis of files that have changed.
To access this feature, all you have to do is right click on the file/folder and select "Restore previous versions. “What it does is that it lets you see the file as it was some time ago and thus helps to restore it by just dragging it to the desired folder and selecting 'restore' or just click restore if you want it to go to the original location.