When Sierra came out, I tried to upgrade to it, but my monitor just showed nothing. When I used CMD+R and restarted from there, I could access Mac. Eventually had technician reinstall El Capitan, because no advice I read helped. Tried upgrading to Sierra again (using the Sierra button that sits on my Launchpad for a year) and this time it got even worse. I can access CMD+R, but restart from there has no effect. I use Philips minotor, where power button blinks when it is asleep. After upgrading to Sierra, it had same effect - monitor seems to be asleep, while Mac is running. Used back up to revert back to El Capitan. Sierra has brought nothing but troubles. Now even El Capitan seems bit slower than before. I hope nothing got damaged during Sierra upgrade and back-up restore? Any ideas of why this could have happened? Is it due to monitor not being Apple hardware? Sierra just not working with my Mac Mini?