I have been editing a movie for hours, now, and iMovie has decided to stop working. I am able to skim the raw video that is displayed on the bottom, but my edited video on top does nothing. I can't play it back in the preview window, either while skimming or selecting whole clips to play. It will not open in full screen, either. I have restarted my computer, and I have repaired disk permissions. This is a very important movie, and I do not want to start over. What steps can I take to fix this problem and, more importantly, make sure that it doesn't happen again?