Farhad,

Most important thing ... stop using the computer immediately. Any activity of hard disk might affect your ability to recover data.

There are several utilities which will attempt to examine the data blocks directly to recover data. I don't know of any that is perfect.

I did a fast search using keywords "unformat recover" and came up with the following site:
http://diskinternals.com/fat-recovery/

This does not appear to be a free program. I can see you can download for free but to repair you probably have to pay to get activation key. Perhaps further searching might reveal a free utility, I do not know.

I cannot personally recommend or vouch for any, not having used any such programs in years.

Regards

Art