Windows Movie Maker clips disappeared?

Hello, I had been working on a project in movie maker using .wmv clips downloaded from YouTube. I hadn't done anything to it for a few months but opened the project today and found that all my clips appear as red X's, and are apparently 'missing'. I haven't deleted or moved the files, and I can view them as .mp4 files, but cannot find the .wmv versions that I had converted and was using previously.

Any help or thoughts appreciated as I'm very upset to lose the work; it was the result of many hours' blood sweat and tears!

Steph