My Canvas went black

I was just editing along switched to a different clip in the time line and the canvas what black. When I double click the clip it shows up in the viewer just fine. I opened a new Sequence same story, black canvas opened up old projects same thing. Re-started the computer....same thing. I can see that the clip is in the canvas I can see the wire frames, move the clip around and see that it is moving in the viewer but just a black screen in the canvas. Also if I try to chance the back ground in the canvas to white or checkerboard nothing happens.
What did I do?!? Any ideas?
Thank you
Nally