Our Mark II stopped working completely after I tried to boot it with the Mark II Dev Kit’s USB drive. When turned on, only the fun started working and nothing else. Apparently, some images break the Raspberry PI (https://community.openconversational.ai/t/mark-ii-stopped-booting-fans-at-max/12726, https://community.openconversational.ai/t/mark-ii-wont-boot/12687). I confirmed this by opening the Mark II, removing the Pi and trying to turn it on with a normal Raspberry Pi image in a SD card, it just did nothing. Also, I put a new 2 GB Raspberry Pi 4 in the Mark II and it worked.
I later learned that the Raspberry Pi got broken because the other USB drive image replaced the Pi’s firmware with an incompatible one. So I decided to try the Pi firmware recovery process described in https://www.fastoe.com/blog/rpi-4_400-eeprom-recovery-guide (but with the most recent image from https://github.com/raspberrypi/rpi-eeprom/tags), and it worked. The broken Raspberry Pi was working again. I put it back in the Mark II, and it also worked.
The problem is that the process of removing the Pi from the Mark II and then putting it back involves some unscrewing and other things that are not trivial… But it is easy to remove the 2 front screws of the Mark II and remove the front cover and the screen, and with this you get access to the SD card slot of the Pi, so it might be possible to recover the Pi’s firmware without removing it from the Mark II. However, I have not tried this, do it at your own risk.