targeted adjustment brush has stopped working

The targeted adjustment brush has suddenly simply stopped doing anything. The "target" circle is still there, but when I apply it, instead of, say, slowly wiping out the bright red of clipped highlights when touched with the brush, the reds everywhere in the image instantly disappear as soon as the brush touches one part of the image. But when I go back to the basic panel, the red/blown highlights are all still there, completely untouched. (Furthermore, on the targeted adjustment page, the mask doesn't show up-- presumably because the brush isn't working.)