Whenever you save or the auto-save saves your script, Final Draft makes an additional date- and time-stamped copy (i.e. 201607041730Scriptname) and places it in another folder for safe-keeping. In this example, the file named Scriptname was backed up in 2016, July 4, at 130 PM.
See these articles for information on accessing the Backup folder:
To be on the safe side, in case the backup is damaged as well, you will want to rebuild the file so you know you're working with a healthy script. Click HERE for information on how to rebuild the file.
IF YOU DON'T HAVE A BACKUP OF THE FILE
and you have Microsoft Word: