[Undo Brush Tool, History Snapshot]

I have exactly this problem; I used the brush ages ago on a different picture, and have not been able to use it since.  It calls itself Undo Brush Tool. I've tried the method advised above; the camera symbol goes white, but nothing happens.  Also tried Control+ Z (as advised in the Edit section).  The Paintbrush button still says Undo Brush Tool. Please help if there's a way out of this! Thank you