Mac OS Monterey Update Stuck

I tried to update to Monterey (I forget which version) two nights ago, and has been “updating” since. The update is stuck - black screen with apple logo and progress bar about 1/3 of the way across. Every few seconds the screen goes black, then the apple logo and progress bar reappear until the progress bar is about 1/3 of the way complete again, and then the screen goes black and the cycle repeats. I can power off my Mac, but when I power back on, it resumes the same cycle. Not sure how to stop the update, and get back to being able to use my Mac again - please advise!