Just to check a couple of easy things: Open your project to see whether somehow your video track was dimmed – say, by soloing, turning off the visibiity, disabling.
The way you describe this, the only other causes that come to mind are that either the path between your project and the Capture Scratch files was somehow changed or that a codec has gone missing in your QT Library.
As for a path change, my limited (since I seldom do reference movies) experience has been that QT Player will complain that it can't find the necessary file – as opposed to just showing a black screen. The black screen with audio only is what I would expect if I copied the reference movie to an entirely different computer from the one I created it on.
You could the codec is still available by going the the Library>QuickTime and that it's functioning properly by exporting a short section of your sequence as a self-contained movie and playing it.
Russ