I seem to have exactly the same problem as Laura.
I've taken on board one of the answers suggesting that it might be the player. But in my case, I've had 4 video clips about 500MB each that played back fine individually, but when integrated into the production as a single movie, nothing but an empty green screen for the most part, with a few seconds of footage before going green. So I'm wondering if it is a question of file size (more than 2GB in the final production).
The format that I produced it in was high-definition MPEG-4. This was also the format of my original source files.
Also, I'm a complete newbie at this, so if the answer seems obvious to someone, please be gentle with me
PS... while I'm here... I'm a little disappointed with the post-production change in quality on the smaller 500MB files. Is there a way of minimizing the distortion/quality to match the output specs with the input specs? I'm thinking of getting boilsoft file-splitter to circumvent this, given that their file-splitter does not reprocess the file in the splitting process (the output file format matches exactly the input format).