Screen goes white when I hit pause/stop

While logging subclips, I thought I was filling in the name when I looked to see that the viewer had turned white. I was holding down the shift key to write a string of capital letters, when I must have activated something that makes the screen turn white when I hit K or Space Bar to stop play. I searched the menus and found nothing. I quit out of Final Cut and openned it up again, but had same problem. I shut down computer, waited a few minutes, turned it back on, and openned final cut, but again had same problem.

What did I do?