ticking remote management has frozen access

I have been checking out an old Mac Mini controlled by screen sharing on my newer Mac Mini running Big Sur. Everything was fine until I went on to sharing preferences of the old computer and for some reason clicked the tick box for 'Remote Management'. Now I can no longer control anything on the old computer, including reversing the tick box! I restarted from the on-off switch and can see the startup log-in but can't type anything in the box. Nothing of the old computer shows up on the new computer's location in the finder apart from the computer name so I can't get to its system preferences again that way. I assume that remote management has somehow blocked me but don't know how to get rid of it. Help appreciated!