The light turned off in one window

I was clicking through stuff fast and my finger slipped, and I’m not sure which key it hit, but I was in the lower left-hand corner of the keyboard, and then most of the preview lights in the left hand window (the one my cursor had been over at the time) turned off. The other window was fine, and I looked in the System & OpenGL tab and the light was still on there… What combination of keys did I hit? How do I make it go back?