Using Layer Mask in PS CS6, the white selection using the paint brush stoped working while the black selection still works. The trouble started after using the ALT key and clicking the Mask icon. In other words, I can't toggle back and forth.
Two years later and no obvious cause and no obvious fix. I probably pressed a control key that changed a mode that is not made plain. Thus it is my fault that my fingers stumbled. It is a *SOFTWARE* problem that the fix is so non-obvious. Please, please, please, Adobe: add a "things aren't working" menu that can help diagnose these issues. Meanwhile, I need to be able to use my paint brush. And, yes, it is in "Normal" mode -- not "darken" or anything else. Also, I right-clicked and reset the tool and even a reset all tools. My brush does not function.