I cannot see the terrain brush anymore.

In my case, the blue brush doesnt appear on the terrain once i put my mouse over it, after try many things i discover what is wrong.

In the Terrain Collider, the TERRAIN DATA field was empty, once i select the terrain data, my brush appear.

This occurs because the game uses the terrain collider to calculate the brush position, when there is no terrain data in the terrain collider, the brush doesnt collide in anything, even if the TERRAIN COLLIDER is active. See in the image below, the red square i do around the terrain data field, this field must be filled (sorry for the bad english, i hope that this help you guys xD)

It might be late to answer but you have to switch from Debug > Normal mode. Just right click on terrain under Inspector and change to Normal

Check if the terrain collider is checked on , if it's off the brush will not be shown because there's no collider.

Helloes

this is quite old, but I did not manage to find my answer on the web, although I found out what was going on

I had changed the layer of the terrain, and as the scene got more complex, I disabled all the layers, temporary changed my terrain layer to "Water" and then enabled that one only.

Unity was drawing the Brush in the "Default" layer, since this one was disabled, it was not showing.

Just make sure you have the "Default" layer enabled

For future people, hope this helps