public beta tester here, with same problem..my 2014 mac mini went to download the latest version and screen has been black for several hours now...i hope it's temporary, as i just bought this kit a couple months ago...i can't abide a brick...i can't get to the desktop to do any of the issue resolving methods, below.. late 2014 i7 3.0 GHz 16 gigs ram

just zapped the parameter ram....no bueno...