If your on windows 7, you can use the previous version feature. This allows you to roll the file back to a previous version.
1. Right click on the file and choose Properties.
2. Goto the tab that says Previous Version.
3. Select a version from the list box, you will need to pick one with a date you know was working. Click on Restore.
NOTE: If you do not have a list previous versions, you probably didn't do any backups or updates or created a restore point during that time. Or you might have it system restore turned off.
If your on any other system, you have to back up regularly. If possible with a backup system that can save multiple versions.
This is how it looks like as an example...