Word is pretty stable though very long or complex documents do rather push it.
I think you have lost the edited work this time.
Have you tried Tools > Options > Save and setting the time between auto saves?
There is the option of Master documents (which I have never needed to use and don't understand) but imagine this is a way of coping with smaller chunks of the whopper at a time.
Word may have been cut off while it was saving or closing and simply lost it.
I have had this happen in other apps where RAM was at a premium because I was doing a montage or playing with all sorts of imports into a different application.
Whatever, when the smoke has cleared all the recent work has disappeared.
As grumpyguts suggested Save frequently and if it is that important "Save_As something else now and then.