So... laptop is usually set up with 2x slave screens.
I obviously had the serial monitor on one of the external monitors.... because when I tried using just the laptop today, the serial monitor was nowhere to be found.
Press the Windows button to show all the available screens (or whatever they are called) and you could see it, but how the heck do you get it back to the main screen?
I messed about for 15 minutes before giving up. I thought it should automatically return to the main screen.