Except just now, Movie Maker has decided that the video that half my clips are taken from is now 'uncompatible'. It's been working just fine until now, and all the clips have been working, so why has it decided to stop working now? The dialogue box is telling me that the file is either corrupted or in a formal that Movie Maker doesn't recognise, except it DOES recognise it, since it's been working for the past two days.

For those unfamiliar with this problem, the clip appears as an error symbol. When I hover over it, it displays the details and tells me to double-click to fix it. When I do, it asks to remove the clip because it is either corrupted or unrecognisable. As far as I know, neither of these is the case.

Why has this happened? What can I do?