I don’t know what happened, but when opening my project a while back, I noticed that I could no longer paint textures or paint trees onto my terrain.
When I select the texture and hover my mouse over the Terrain, it doesn’t even show me that light blue brush zone. It feels like I somehow locked the editing of the terrain.
Create a 2nd terrain in the same scene, edit it and then resume editing on the original terrain.
Broken GameObject?
Create a new Terrain GameObject - then in the Inspector, look for the “Terrain Collider” component and set the “Terrain Data:” field to reference the terrain data used by your original terrain.
If that works your original GameObject may have become corrupted somehow.
If the terrain still isn’t editable then the problem is likely to be the TerrainData…
Broken TerrainData?
If this is the case and you really want to retrieve an editable version of your terrain - you will probably need to export the terrain’s maps and data and then import them into a new terrain (for splatmaps, trees and details you can get a 3rd party extension that will add the import/export features to Unity. (There are several that can do this and some of them are free)