1) Great discovery:D - before this happened, I opened the map where I had saved the video, but while I was creating the video, I opened it immediately. However, I'm not sure if this is the main reason.
2) Small discovery:D - before most of the video was deleted, I was opening the effects to see which one I needed.
3) The Cameras' Revenge:D :D :D - at first, I created cameras (frames that the camera moves between) named Camera[number], but then I added a monster that I needed for the plot – the one dancing with the wind – and, to avoid image problems, I replaced all the cameras with new ones with different names. Since then, I've been creating cameras named Cam[number], but I didn't delete the old ones. The missing dancer is the main clue supporting this theory, but I think the real reason is:
4) - The Cameras' Revenge killed the video after the Great and/or Small Discovery:D :D :D :D :D.