After a morning floundering about, I finally figured out what it was. I had unplugged my monitor DVI cable last week and for some reason Premiere seems to have thought I was now playing back video on an external monitor rather than the source and program monitors. All I did was turn off the computer, pull the monitor jack and plug it back in. Now premiere is playing back on the source and program monitors.
That was after, of course, rebooting the machine, updating the graphics driver, cleaning the media cache, posting here, and scouring the web for help. Sigh.