Preview window goes black

hey everyone,

i have final cut pro on my macbook running mountain lion, it has worked forever and although it takes some time to start, while it's running its great. I had to use it for a project today because iv had another problem on imovie. Anyway, im importing mov and mp4 videos from my harddrive, into two seperate events. The first event, had the same problem, but after i started using a movie, they all showed up. the problem is that they are on my library and in the event, but the preview window shows as black. and if i pull one of these videos to the time line, the audio plays but the image continues black. I cant understand what the problem, is and why one of the events turned ok and the other didn't... any ideas? what can i do?

Thanks

Gui