Masking tools stopped working

Hi

Some of my masking tools have suddenly stopped working.  I've searched the previous posts and done as suggested, but still have the problem.  At the moment, the brush tool no longer works at all; it worked fine until a few days ago.  

I'm on the latest version of Lightroom, Version 9.0.  I've downloaded & installed the latest update to the graphics card.  

Would anyone know how to fix this?

thank you in advance!