Is it only that one file or all your files? You said you saved often. Are all your saves under the same file name? If so, then for future projects, I recommend saving your projects under a different file name ever so often.

I usually save about every 5 or 10 minutes and I'll save my project under a different filename at least 3 times a day. I usually end up with about 15 different files for same project. Examples: House1_1, House1_2, House1_3.
-or-
House1_wallsDone, House1_windowsDone, House1_roofDone

I'll do some research to see if I can find anything helpful that might apply to the error you're getting.