Failed saving Word doc.

How is it possible that the word file didn't save after multiple times saving it? I was working for 3 days on a project on Word and every time I save it before closing the laptop. I didn't close this Word document in these days. The day I got finished with my project and saved it, I finally closed that document. When I opened it was back to the work from the day before meaning 9 hours of work is gone. How is this possible???????