Some clips won’t play

I don't understand, why the clips, I have added to a Movie Maker project, don't work. All the clips in the video are of the same format, and I have recorded them myself. But suddenly, while I am editing, more than half of the clips don't work (show just black screen). When I double click on them, a box shows up, that says I have to delete the clips. I obviously don't want to remove them, as I have used more than three hours to get them right. Please help me before I go mad!!