This is my first time of using Windows Movie Maker so, I'm not sure what the problem is. What I want to do is to make a video with 5 short videos, each lasting 3-4 seconds, with a background music. But when I edit some videos three to four times, like adding effects, they show errors. It's not that I can't play my video anymore. It's just I can't edit it. So, how can I fix that kind of error?
Chances are, the corruption was just caused by a hiccup in the program while it was placing the effects into the project file. In my experience, all versions of Windows Movie Maker beyond the one bundled with Windows XP SP2/3 are incredibly buggy and should be replaced with a better video editor. Problems like this are ridiculously common in the Vista version of Windows Movie Maker.
To answer your question, you should try closing and reopening the file, and if it is still corrupted, you'll obviously need to start over. The issue may be that Movie Maker inserted malformed properties into the project file, causing Movie Maker to not be able to parse it correctly.