Thanks, Hans, I hadn't thought of that possibility, but Matchboard if it is floating is not floating anywhere I can see, nor is there any way I can find of unfloating it.
I found however that if I closed the project and CT and then reopened CT and any other project, then closed that, and reopened my problem project, the Matchboard magically reappeared. So that appears to be one solution at least,
Not sure which OS you are using, but FTR, if you're on macOS: press Fn+F3 to show all panes. There must be a corresponding keyboard shortcut for Windows/Linux.