My Macbook pro gets stuck booting after an upgrade from Catalina to Big Sur that failed

What MacBook Pro sub-model, the bit with a year? If 2013 or 2014, please see this Apple article:

If you can’t install macOS Big Sur on certain 13-inch MacBook Pro computers from 2013 and 2014 - Apple Support

Apple have now blocked Big Sur installer downloads for 2013 and 2014 MacBook Pros pending resolution of some problems..