Conversations involving multiple participants, allowing collaboration and information sharing in Teams
Thank you Hummelmose, that is wonderful, and has fixed the problem! I'm not bothered why, as it is most likely an accidental key press (probably alt and something) but at least I now know how to fix it.
Forget reboot.
I tried the following from a suggestion:
Right click on teams logo at the bottom:
Select X Quit to get out of Teams from the options shown:
Go back into Teams and the problem was fixed for me.