MacBook Pro Stuck After Updating OS

In an effort to do my taxes, I had to upgrade my OS. I did so; process was pretty smooth/normal; UNTIL the Mac restarts. The login screen appeared; I enter my password…after a lot of spinning, I get a pop up window that asks for me to “Enter the Apple ID Password for _____”. I do that; and it spins for over an hour, then I get “Unexpected Error Occured”. I have done a hard reboot (pushing down power button) with no effect. I can’t get it to start in safe mode. Anyone have any suggestions?