Okay, if there are no .umap files, that means that there was not a backup created. Typically, this sort of message would mean that the map file has become corrupted. Resolving this usually involves either grabbing one of the backup files (which in your case there are none, unfortunately), recreating the affected map, etc. There are some suggestions in this forum post that might help: Failed to open map file. This is most likely because the map was saved with a newer!! - General Discussion - Unreal Engine Forums!!
Otherwise, it just seems like a corrupted map file. The best way to prevent data loss is to look into using Source Control. You can find our documentation on Source Control here:
Have a great day!