My adjustment brush section of LR 5.7.1 for PC just "quit"

[Nikon D90 Gent II]

which puts me in a pickle.

The adjustment brush/sub-edit section has no effect on my editing anymore.

The show mask overlay (painting in red) vanished/became invisible.

So I cannot tell what section is being edited and can't see any/the area selected.

What did I accidently do to cause that? All input/suggestions welcome.

Please help restore my ability to make intelligible and visible edits!

Thanks in advance.

[F. McLion]

Showing/hiding the mask overlay: Hit 'o'

Changing the color of the overlay: Hit Shift + 'o'

What is your OS?

[Nikon D90 Gent II]

I use a PC running Windows 7 Professional.

Version is desktop 5.7.1.

Thanks.

BTW, is there a on and off switch for this brush?

[Nikon D90 Gent II]

Is your suggestion the same for PC or MAC?

[F. McLion]

Most of the shortcuts are the same on both platforms.

[F. McLion]

What do you mean with on/off?

Maybe this: