Most likely your file has been corrupted. If it is, you're kinda SOL when saving with .mb. With .ma, you can (sometimes) recover your information. Hopefully you have backups.
I opened the file in a hex editor (allows me to take a look at binary files) and there is absolutely zero information in the file; it's all zeros. Sorry, but it's gone.
Don't think this thread is in the right place, but I have exact same issue. My file is attached. Please, tell me it's not all zeroes I've been working for a week on this and I made backup ages ago.