Green error screen during playback and render

I just downloaded FCPX a few weeks back and I've only used it for 30 second bumper videos so far, but when I tried to use it for a longer 2 minute video, the transitions and overlayed clips all of a sudden turned completely green. The audio still plays perfectly. And footage without transition or overlayed clips plays back fine. It does the same thing after render. The final product has the same green screens in them.