Premiere stopped playing any video (or showing me any video)

I've been editing all day in Premiere Pro Beta, but just in the last half hour it stopped playing any video. When I click on a video clip in my bin, I can't load it (the preview window is just gray), and when I click my playhead onto the timeline and hit play, nothing happens. My program window is just black, no matter where I place the playhead on the timeline.