Today I was working on a mask, and I took the arrow tool and was trying to grab a corner handle to make it smaller, and after I clicked and dragged, (I missed) it made a little grey box. All the comp disappeared until I dragged the little box to the edges of the comp. Now the box stays lit up, and I can see its handles all the time. I cannot delete it. What the heck is it and how do I get rid of it?