Gradient tool only works half of the times

Hi all,

This is definitely my fault, but the gradient tool only works half of the times. I must have pressed a key on the keyboard by mistake and I can't revert the tool back to normal.

When I use the gradient tool (black and white) it only woks if the mask is black. If the mask is white nothing happens.

Also, if I use the tool directly on a layer (not the mask), it will only apply the white part, not the back. It's as if the tool is white and transparent, when it is clearly white and black.

Thank you for your help.