Okay, I am going to reveal the full extent of my Photoshop-ignorance but so be it - I need help 

My work flow is pretty basic: I used adjustment layers to adjust levels, curves and occasionally color balance and saturation.  For isolated objects I sometimes use a brightness layer, crank the brightness way up to get a nice white background, then fill with black and use the brush tool to reveal the brightend areas where desired.
At least that's how I did it until this morning when all of a sudden the brush tool won't work to reveal the brightend areas anymore.  I must have something stupid  (the layer isn't locked) but I have absolutely no idea what it might be.
 
I guess I better invest in a photoshop class but until such time - anybody any ideas what to do?  I need to keep uploading on shutterstock - it's just way too much fun to see the sales roll in to stop!

Thanks everybody,

Tina

Duplicate the original layer  , brighten your background to what you like , add a layer mask to new layer. Check the  OPACITY  of your brush and the color you paint with ( black ) and it should be fine.