I've encoutered this problem twice already where one of my clips just shows GREY and only the audio plays (in the source monitor), yet the stuff I've already edited with using that clip works perfectly fine on the timeline, so the clip at one point did work correctly and sort of continues to do so except I can't view the video in the timeline to continue editing with that soruce. It doesn't show the clip is missing. It's just greyed out in the source monitor with the audio from that clip working.
The first time this happened I was already done with the project so I didn't bother doing anything. When I preview the playback within the cliplist. It goes grey/black too, then eventualy resorts back to a icon that isn't black/grey.
I did find this thread... but the issue I'm encountering has nothing to do with the timeline and I've restarted media composer already.
http://community.avid.com/forums/p/97795/689263.aspx
I'm working with r3d files that I trancodeded within MC 7 after AMA linking them. The clip itself is a linked to a MXF file. I have already deleted associated files to the clip, retranscoded the AMA. It's relinked again and still doesn't show up on the source monitor.