still playing with it...after two trips to the apple non repair store but working so far catalina induced I think

[lasloduncan]

cant find it now but it is "apple support logon screens" system hangs on boot up....hold power off for ten or so seconds should power off power back on with cmd and r key that will hopefully (worked for me imac catalina) load the restore screen ..restore catalina from here...here it is

If your Mac starts up to an Apple logo or progress bar - Apple Support

Your Mac shows an Apple logo when it finds your built-in or external startup disk. As startup continues, you should see a progress bar, with or without the Apple logo. This screen might alternate with a blank screen several times.

support.apple.com