Get halfway through a model and find it's an unworkable mess? Can't add edge loops where you need them? Can't subdivide a mesh properly? If any of this sounds familiar check this course out.
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.