Lost Document

Hello,
I was working on my paper for my Master's Class this morning. I had pulled up my last submitted paper and was making modifications to it and was getting ready to save it as the new paper when my screen went back to my desktop and my paper was gone. I have looked everywhere in every file and cannot find it. The paper that I used to make the modifications to is the same as it was, as if I had made not changes to it. Do I just need to start over or can I retrieve it somehow?
Beth
I have windows 7 and the paper was in the word format.