I've already rendered this scene several times, the lighting is all working fine, then suddenly, the renders are black. I didn't do anything except adjust the angle and intensity of a single light, as far as I know. And I adjusted the intensity up, not down. Anyway there are 6 other lights. What gives?

The only thing I notice that's different is that when I go to the perspective view of any of the lights, there is a box on the screen that basically shows a cropped view of the viewport, with a 1:1 in the upper left hand corner that wasn't there before.