I inadvertently touched the comp window with the arrow tool.
It created a mask I do not want (comp window went black and added a white outlined box which seems to be a mask that can be expanded to reveal the layer I was working on). I can't figure out how to undo or eliminate the mask.
I hadn't saved my project for a while so I don't want to close/reopen it and lose what I've done.
ROI can be a really useful tool, too. If you have a really heavy comp, you can enable the region of interest while you're working to render only the portion of the frame you want to focus on, instead of having to render the whole frame.