There might still be a chance that you can recover the .pst file.
When a client deletes a file from a remote resource, the deleted file will be place on that computer's recycle bin.
Get yourself a copy of Recover4All. It is a data recovery tool used to recover information upto sector level (even if you lost the partition or formatted the hard disk drive, it can still recover data.) DO NOT DEFRAG EITHER THE WORKSTATION OR THE SERVER. This will destroy any chances of recovering information.
Good Luck