First day using Noteperformer. When I went to close the file I got the spinning beach ball. Force Quit, and now Dorico won’t open—can’t connect to audio engine. I tried uninstalling NotePerformer—still no dice. Can someone help? (On Mac Sequoia silicon.) Thanks-Peter
Never mind… restarted my computer and it’s working again. 
The problem you’re facing is that for some reason, the audio engine (which is a separate app from Dorico) is stuck and has not quit. Open Utilities>Activity Monitor, and kill VST engine (which should appear as stuck)
Relaunch Dorico after that (leave 5 s to make sure everything is in order before you relaunch Dorico) and it should start.
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… which would explain why my restarting my computer fixed the problem. Thanks—this is exactly the info I was looking for! Since it could happen again…