It is most likely a codec from a camera or output source which is not installed on your system. I just ran into this last week - the footage was from a Blackmagic Ursa camera and it would only display green. Luckily I was only using the footage as reference for a documentary graphic design package, and I was able to pull stills from it.
The way in which I did this - and the way that you can check whether this is your problem - is to try playing the footage in VLC Media Player. It will play just about anything, plus you can probably identify the codec that's missing by looking at the properties in VLC. You can also transcode the footage in VLC to something compatible with your system, just in case you can't get the missing codec. Good luck!