So I was working on what was originally on online project, saved to disk and disconnected from PLC. Put the cursor on a rung I wanted to comment and hit Ctrl-k. Nothing. Went to the tools menu, and it was grayed out.
Turned it off and back on again

(shut down DMD restarted and reopened the PRJ), and still no joy.
What could disable comments? Don't think I've ever seen that before. Not a huge commenter, so it might have been there a while, or not, I have no idea.