My brush tool is not working (not even the grab one). I even tried the clip end solution, changing the values of clip start and clip end. Does anyone here have idea about what´s happening and how to fix this?
You probably need to add more geometry. Say for example you are trying to sculpt on the default cube. Select the cube, and press tab to enter edit mode. Notice how the cube is composed of just the eight corners. This means that the sculpting tools only have the 8 corners of the cube to play around with to simulate the brushes, which obviously isn't enough.
Now try adding and applying a Subdivision Surface modifier to the cube. Ignoring how it changes into a sphere, the sculpt tools now work since they can edit the existing geometry.
You can try enabling "Dyntopo" meaning Dynamic Topology in the top right-hand corner. This will add more geometry as you sculpt, but be careful as if it starts adding too much, your computer will lag.
Alternatively...You've turned the brush strength down to zero.