I made a clean install of macOS Catalina on my 2018 MBA to test the system, and all went well. Today I tried to update Mojave to Catalina on my 2018 Mac mini. Initially, the install was going well, but at one point the screen went black and remained that way for half an hour. I tried resetting the machine to no avail. There is no video output on the HDMI port, although the LED indicator is working on the Mac mini. It won't even boot into recovery mode or give me an option to select a startup partition – the screen remains black. I've already tried resetting SMC several times, but that doesn't help.