I understand your desperation. It caused me to go back and double-check my previous statements, based on a discovery long ago. My camera is an early SL1. It does have a small (4x7 mm) rectangular switch centered between the rubber part of the eyecup and the hot shoe. I did not find it documented anywhere and have no idea of its function(s). It might relate to a 90 degree viewer attachment which I know nothing of. I know you have a different camera. In desperation, maybe yours somewhere has a switch related to whatever the function on mine is.
I had this problem too. If the Canon eyecup (the rubber peice that slides on and off the viewfinder) slides up a little bit, the screen will not work. Just push the eyecup down and into place and the screen will function again
12/05/2023: New firmware updates are available.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.