Just to update this one after several PM's, the project file is probably corrupt, so try using a backup file.

Go to Load a project. When the file manager screen opens, change search extension at the lower right from *.mmm to All files *.*. Then you will see all files. Your backup will have the same name but with "_BAK0.MM_" appended. By default and unless you changed it, MMM creates a backup every 10 minutes, starting from ...BAK0 to ...BAK9 and then starts at 0 again. You have to look at them all and pick the one with the latest creation date. If it is corrupt, try the next latest, and so on.
 

When you are doing a project, I recommend changing the filename from time to time, especially before doing a difficult of suspect operation. Do a Save, then Save as and append r1, for revision 1, r2, r3, etc. or something like that. This way if something goes wrong, you can easily go back to the previous revision. The backup files are there as a safeguard, but as I have shown, they rotate around and a previous earlier version may not be available.

Good luck!