Traindaddy Rob ment the A or the U terminal, I believe.
I’m thinking it’s more than just the terminal with the lug nut on it, I’m wondering if it is a broke wire in side, ( ie connection to the A lug, as it does happen. ) personally if you haven’t opened to look you don’t know what it could be. It actually could be a lot of things, none of which are real hard to repair.
Looking at the back or the front? “A” handle or “D” handle? Did you try a different “U” post(all four are electrically the same, sometimes they will become disconnected inside the case, and it’s always the one at A or D that comes loose first) with that handle?
FOLLOW-UP: A “whatchamacallit” that was supposed to be connected to a “thingamjig” came off. It’s been reattached and all is ok. Thanks for all your advice."