Here's a free program called Mac Data Recovery. http://www.macintosh-data-recovery.com/mac-undelete-files.php?gclid=CITqmuqI7qQCFYft7QodcxwW0w
If you have written more data to your hard drive, the old files may have been overwritten. The sooner you attempt recovery the better. If possible download the recovery program on another Mac and do the recovery attempt in Target Mode. Remember you will need a destination disk to store these recovered files and that it may take several hours. I like to use an external drive to recover to.