No sound to my video all of the sudden in Window Live Movie Maker

My end movie is over an hour long.  In the construction of it the sound worked fine.  I would preview it while the rest of the videos were downloading and once I had all the videos in the window that I wanted, in the order I wanted them I walked away as it was going to be a while before it was ready to even save or publish.  When I came back to check on it, it was done downloading but there was no sound.  I checked all the systems and they are on.  I did turn off the screen to save power but that shouldn't matter should it?  In the edit tab the video volume is set to high and nothing else was done to the video while I was away, at least not that I was aware of.  Any ideas what's going on? I took me a good hour and a half to get this video together I don't want to have to do it again! THanks!