Ok I was having a major issue with my Mac mini after installing Catalina . The video card would not boot up unless I did the reset by hold control command r p .
I used time machine to restore to Mojave right before I upgraded and the problem persisted . Just for chance I plugged my hdmi cable in to usb c to hdmi adapter and plugged in . It recognized the display and work normally at boot up. I think what ever happed in Catalina needed the system to detect a new monitor.
i reinstalled Catalina and it took about 45 mins to set every thing up and is working fine. I powered down a couple of times and it is running great . In fact the system seems faster .
while I was at it I put usb ssd in place for time machine. It took time machine two hours to restore from the old ubs2.0 drive .
just finished backing up the system and running clean my Mac .
there is joy in Mudville again