Question about Outlook Express

I recently just made a movie in Windows Movie Maker 2012 and after I had made my final edits and after I published to YouTube, I saw that the YouTube video screen was completely black, but the audio still played from my movie. The problem wasn't YouTube because i checked other videos and they played fine. So i went back to see what the problem was, and after viewing the movie in windows media player, it was the same problem. The video playing had no picture, but i could still hear the audio. I saw online that this may be a compatibility or codec problem? But i do not know specifically what the problem is or how to make it right. What should i do to fix it? And can i still can fix this without losing all of my work or having to do it all over again?