I now get a blank (white) screen in the Viewer. Can still drop video from the viewer into the canvas, but don't see it in the viewer. Please help!

It doesn't seem to matter what project I'm working on... the picture in the viewer is gone. All I see is a blank (white) screen. If I put the cursor over the viewer and drag to the timeline, the clip will appear on the time line (and in the canvas) but I don't see it in the viewer. I usually edit my clips in the viewer and not on the canvas. Can anyone explain why this might be happening and how I might fix. I'm suspecting I must have messed up one of the settings but I'll be darned if I can figure out which one or how to fix. Please help!