In certain instances (I'm not 100% sure which), I've found that the Terrain objects are considered gone by the program. So, if I go to add something in the terrain objects, TG2 gives me a message about needing to have terrain. When this happens, I go to the node network and see the original terrain is intact and all connected, but the Terrain under the file menu is empty. If I add a crate shader (as an example) to the Terrain node group and attach there, nothing shows up in the terrain. Also, if try to generate a heightfield, TG2 goes through the motions (0% to 100% complete) without doing anything to the terrain.
Since I have so little information, I'll continue to pay closer attention and fill this in as I know more.