I had a quite similar issue with my rather new 90d. It stopped working all of a sudden, but I found a solution for at least my issue.
On my 90d only the small top monitor kept working, the large monitor did not turn on and I was not able to change any setting like shutter speed or aperture and was not able to take any pictures. After couple of seconds after turning the camera on even the off switch did not work, the top monitor stayed on even after turning the switch off.
I tried removing the battery, lens and SD card, with the switch turned on I kept pushing the shutter button for around 10 second and left the camera for 20 minutes. But that was not enough, the error still was there.
I did the same procedure but left the camera sit for over 48 hours and that worked for me. I then set the switch to off again, put the fully charged battery in without SD card and lens and the camera switched on. I was able to make a camera reset of the settings in the menu. So far the camera is working without any issues. It has been couple of days and over 50 photos.
Don't know if 24 hours would have also been enough, but I wanted to make sure and had the time.
Maybe this information will help somebody else with a similar issue.
Apparently there was some sort of hick up in the camera software while I was taking photos, maybe I was a bit to quick with pushing different buttons or just some sort of random code malfunction. This error was stored in the RAM of the camera and for the RAM to reset all the voltage that is stored in the capacitors has to drop to zero. This needs some time. I found recommendations for periods of 24-48 hours.
Cheers!
