Yes - I wondered if that was the problem.
I know approximately what layers do, I see construction lines are on their own layer by design. I've also the impression that if you group things into blocks they become a layer of their own. However I don't understand layers enough to risk making multi-layer drawings deliberately, so I keep to single layers. These are also straightforward 2D creations too, with the title above the main tool-bar having the legend "World Plan" attached automatically to my own words.
Examining both Blocks and Layers. The Blocks field was just as a black rectangle with a little tool-bar, so empty. Layers show all as intended, one drawing layer plus the automatic one for the construction-lines I'd used.
Program and Drawing set-up displays, as far as I can interpret them, show nothing out of the ordinary.
File Properties with TC off, via the normal Windows file-managing functions. It's not marked as read-only, and I could see nothing untoward with its properties.
Drawing a random little line away from the main drawing so less likely to be affected by any editing on that. That too, is non-selectable.
Re-opening TC having closed it down and turned off the computer completely for the night: no joy. It wasn't some glitch cleared by re-setting. The Select button is as it should be - sharply defined with the "pushed in" appearance: nothing like other tools shown as inactive by being flat, faint grey.
Opened two other recent works over the top of the faulty one: they both work fully, and I can Select various entities on them as normal. No part of the faulty drawing can be selected: outlines, centre-lines, text boxes, random test-line!