After my cri de coeur yesterday, all is back to normal! A Happiness Engineer (God help him IF he ever has to give his job title on a tv game show) got back to me, taught me how to take a screenshot and send it to him, whereupon he came up with the solution, which was to press Control and Plus together until the zoom became 100%. It did, so I daringly closed the window to see if I could get it back. YESSS!
It is weird that it worked, because every other site I opened had been normal while my blog was not. After the Control Plus operation ALL sites are normal.
Thank you, WordPress Happiness Engineer, Ainslie K. you have saved my sanity.