Hi,
First, make a copy of your project and all its files and store them somewhere safe to keep a reference to them and to be able to at least restore to the current state in case things go wrong! Now, back to your project, go to its directory > Saved > Backup > Find your map, copy it and replace the original map in the Content folder with it. Hopefully, your backup file is not corrupted and you should be able to get your work back.
Hope this helps!
P.S. For future reference, always try to back up your work and keep more than one reference to it.