I edited some videos in Movie Maker. I can still play the original videos (MP4) just fine, but when I open the edited versions (Movie Maker Project) I just have a series of clips with exclamation points in triangles. I still need to edit the videos even more. (I have Windows 8.1 and just up-dated Windows Essentials.)

1) Can I fix the ones I've already edited so I can work on them more?

2) If I have to start over, what do I need to do this time to make sure they will play this time?