Well, I was just coloring in some lineart I'd drawn, then the paint suddenly quit working. I've closed the program and started it back up again. When I go to the "Undo History" It still tells me I've made a few brush strokes, yet nothing has shown up. I've checked, and the brush settings aren't transparent and I've tested it on all layers. It simply won't paint. Is there anything I can do/download to fix this?

Whoops, fixed the problem. Apparently there was one pixel highlighted in a mask. >.O