If you are getting a message saying unexpected file format. That means the project file has become corrupt. Follow the instructions below in order to recover the file.
Open a blank new drawing.
Go to File >> Merge, Navigate to where the project is saved
Select the project file you were working on and
click open.
Accept any error messages by just hitting okay.
You should see the drawing If the file merge is successful.
Then save the project file.
Make sure to check your project to see if anything is missing and it
may need to be re-scaled. The merge procedure might not work sometimes because
the file might not be recoverable.