This is an odd one, but it may happen to others.
I have two identical TV’s. I had connected a Pi Zero to one and did some configuring on it everything shutdown normally. The following day I connected the Zero to the other TV. It appeared to boot immediately to the desktop but was not really working. The num lock and caps lock lights came on and off if the buttons were pressed but in ever other respect the keyboard wasn’t working. The system clock never moved, and although the mouse was lit up it was unresponsive.
I tried everything I could think of, including re-imaging the SD card, but it did exactly the same. I had given up and was clearing up when I accidentally caught the source button on the TV remote. It was now set to the DVD source but was still showing the Pi desktop !!!?
I rebooted the TV and the Pi and everything is now working properly again. The TV had been rebooted since the Pi was last connected to it. The Pi had also been rebooted. Somehow it had remembered the last desktop !!!
I have no real explanation for this but it’s another one for the check list .