I had the same thing this morning, after a whole night let the update do its work. So a reboot: no progress.Same thing except my progress bar is right at the beginning...
Boot in Safe Mode => that worked. Reboot, no progress, progress bar right at beginning (as mentioned by hllywluis)
So I decided to reinstall Big Sur using the Recovery Mode. After 45 minutes everything worked.
Pfff, this was my first time an update didn't work.