macbook pro not restarting after attempted big sur software update

hello - having spoken to an apple support engineer this afternoon to fix an issue with my macbook pro internal microphone, i download the latest big sur software update as advised. however, having followed the on-screen steps my macbook is now stuck on the 'less than a minute remaining' black screen. i have tried rebooting a number of times to see if this helps but each time the screen starts at the 'about 29 minutes remaining' black screen and then - after counting down approx. 29 minutes - it freezes on 'less than a minute remaining' again. i can't start a chat session with apple support as i can't access my macbook pro's serial number, and i can't arrange another call until tuesday. can anyone help please??? many thanks!