I have an old (2011) Mac Mini, which was running High Sierra. It was running perfectly well, but really slowly, for reasons I couldn’t track down, so I thought I’d try resetting the PRAM. I shut it down and restarting, holding command-option-P-R. It chimed a couple of times.
Now, it won’t start. When I power it on, it chimes and the white LED illuminates, but that’s it. There’s no video output, so the HDMI-connected screen stays blank.
I’ve tried unplugging it for a while and resetting the PRAM again. No dice. I’ve opened it up and re-seated the RAM, just in case, but it’s made no difference. The only devices connected to it are the keyboard and a wired mouse.
Any ideas gratefully received.