anchor is correct. It's not possible to recover a website's source files unless you're in possession of the information needed to upload them in the first place (otherwise anyone could mess with anyone else's website).
If you're simply trying to recover a file from your own website — perhaps to get yourself a fresh start after accidentally losing data while editing the file — your FTP software should allow you to select individual files and transfer them back to your system. I use a freeware program called FTP Commander, which displays all my source files in two parallel columns: one showing the files on my HD, the other showing the files on the my ISP's server. Between the columns are two arrows (one in each direction), and transferring files in either direction is a simple matter of highlighting the file and clicking on the appropriate arrow.
The ability to pull source files back off the server has saved my bacon many times!