I've experienced this for a while now, after a decent period of editing in terragen (mostly when I'm using the node editor, but not always), the buttons stop functioning, I can press them, and they show the little press effect, but nothing actually happens. Sometimes I can navigate between the category tabs, but if I press any button, nothing happens, this is most annoying, because the save feature always goes along with it. I've tried ctrl + s, but that doesn't work either.
Oh, and I can't exit out normally under these conditions, and have to use ctrl + alt + delete, and end the tgd.exe process (it doesn't work if I end the program) and get the usual "This program is not responding" routine.