Sony Vegas 9 Pro will only play audio, and not video?

Sony Vegas 9 pro earlier was playing audio and video for my .mov files. I continued to edit and make my video but it started to lag really bad, the frames would blur together, then as i entered another .mov file all the previous clips that used to work no longer worked. The audio for everything still works just no video. I opened a new file for Sony Vegas entered a .mov file and it was working fine. Does this have anything to do with it being such a large file? I have about 3 GB of .mov files in my current file for Sony Vegas, and have spent about 5 hours editing it. I would hate to start over again, is there anything I can do to get visuals back?