I installed Catalina on my Mac Mini, and immediately lost all screen output. Apple support could not help me, and I was going to have to take my machine into an Apple Store in London, when I stumbled across an answer on a Reddit forum. It transpires that Catalina doesn’t like some 4K monitors. I had to completely remove the power from my Mac Mini, and then reconnect it, and my monitor started working again. Not a great user experience...