Am working on project that includes many short clips. Everything was fine this afternoon. Now when I open the project, some of the clips are fine - but others have no audio and only display a black screen (with the caption if there is one) instead of the video.

I tried adding the original footage into the project again. It worked fine for 1-2 minutes then ...no more audio and only a black screen.  Happens to 4-5 clips in a total of about 20.

Am so near finished with this project...and down to the wire on the deadline. Any ideas about what's going on and how to fix?