How do I solve this audio and picture problem?

I was creating a video, and I had a few videos put in. I was playing my video and checking it for mistakes. Then, everything turned to black, and the audio couldn't be heard. However, it was only the one video (and its sub-parts, that were split and snipped) didn't have the picture and audio. Everything else (including some of the other videos and pictures) were working. 

How do I solve this? Also, why did it happen?