Well the obvious answer is that you recover the file from your latest backup, but I'm guessing that because your asking this question, you don't have one?
If that's the case, then your pretty limited in what you can do. If you have saving of previous versions turned on in Windows you may be able to load an older version by right clicking on the file, go to properties and look at previous versions.
However, more than likely you are going to have to accept that it is gone, learn your lesson and start doing regular backups.