Videos won’t show any more when I work on my projects.

I don't understand why my videos wont show up, all they show is a black picture. Funny thing is I was working on it and everything was fine. I saved my project and went to school. when i opened it again some videos just showed black and the others play. when i finally publish the video they actually show in the finale. But its hard to work on it when i cant see nor hear my videos. Its weird cause sometimes it shows and sometimes it doesnt at all. I dont know what to do. I havent deleted any programs and im sure i have the latest WMM. Any suggestion why it might be doing this and how can i fix it? I dont want to restart my project when the videos are technically fine. Also side note: if i delete the video and put it back on it justs stay black still, i never deleted the videos off my computer and all the videos were working fine until i saved and exit out.