Video not displayed

I created a short (3 Min) video. The clips all show up on the timeline. However, the large display went blank after I applied several filters (sharpen). Nothing shows up and when I export the video it plays back entirely black. What button did I push to make this happen. I tried creating a new clip and everything works fine in that effort. Help!

Also, when I try importing a new source the time line is blank and the video is blank.