Yes, I know the problem. I have it VERY OFTEN when I use Windows Movie Maker. I never found out what to do to avoid that, except I save the project VERY OFTEN (after every thing I do, in fact). I even gave the advice to save very often on my first tutorial, saying that in my opinion, WMM was an unstable software.
That's why I almost never use it. I prefer Adobe Premiere Pro - definitely much more options, we can't even compare, and a lot more stable - but this software is very expensive. It's the better one that exists but it costs a lot! It exists an element version, less expensive, but still!
Anyway, back to your problem, the only advice I can give you is to erase the episode from your collections and try to import it again.
To do that, you click on the "collections" button, and then, the task menu is replaced by the list of the collections(= the clips that you imported).
You right click on the collection that gives you a problem, and you delete it.
Then, you import it again.
If that doesn't work, I can't help. Sorry. I hope someone else will have a better solution, because personally, I never found one. I really don't like WMM.
