what I want to do is "unerase" something and for that I need to use the white brush, which I have always used and it worked perfectly fine.
I might have clicked and something that might have cause it no to work. and I don't know how to make it work again
Clipping masks are a little different from, say, vector masks. They simply take into account the content in the base layer they're clipped to, whether they're black, white or any other color.
To reveal with white, you'd have to insert a vector mask on the base layer and work from there.
ON EDIT: This assumes you have more content in the base layer to work with, so scratch that. You're probably better off just using a vector mask and concealing/revealing that way.
A clipping Layer that has a Mask is very simple.
The Mask is either BLACK or WHITE attached to a clipping layer.
If a "Mask" is White, use a Black brush tool to reveal.
If a "Mask" is BLACK, then use a White brush to reveal.
Clipping is a separate issue. Masking is a separate issue.