I have used Movie Maker many times.  Usually I create 20-30 minute movies. This time my creation is over an hour long.

I created it fully. All the files were "prepared" by Movie Maker before hand and I subsequently did all my editing.

However, the last time I opened the movie (before I converted it to a final MP4 file for distribution), it suddenly started saying it had to "prepare" the files AGAIN for editing. I already did this step, but it was doing it again.  And.... it never completed.  Instead, it told me my files were incompatible with Movie Maker.  Again, this was after everything had worked just fine through multiple editing and saving sessions.

So now, all of my work is wasted unless there is a solution at hand to get Movie Maker to open the file properly as it already had many times before.