Lost progress on a SAVED Movie Maker project – impossible to continue

Hi, this happened twice already and I don't want to waste more time editing video and losing progress again - so I'm asking. I loaded about 3,5 hours worth of GoPro video to Movie Maker in order to cut it and after about half an hour (after getting down to 2,5 hours worth of material), the videos on the right suddenly start disappearing under the cursor. I can't continue working so I turn Movie Maker off and when I load the project, there's nothing - except for the timer, saying the project has 2,5 hours. But no video, no sound, no nothing. It's very frustrating. Happened twice already. Why? And is there any way to recover it? Thanks...