If by "overwritten" you mean that another file now occupies the actual physical sectors on the disk that the file you lost was at, the recovery from a backup is probably your only option. If, however, you mean that another file of the same name has been written to the same folder on the disk, it does NOT necessarily mean that the original version has been physically overwritten. In the latter case, you might rename the newer copy of the file to something else, then use an undelete utility to try to recover the earlier version. Under most circumstances, it's unlikely you'll know if the file has been physically overwritten until you actually try an undelete. It's definitely worth a shot if you have no backup. Good luck!
Joe