Mac Big Sur update stuck on apple logo

Force shutdown even by Power Button. Disconnect all external devices. Connect charger. Restart in Safe Mode. This may or may not allow the process to complete - hopefully successfully.

Safe mode may help you resolve or isolate problems that you’re having with your Mac.

When you start up in safe mode, your Mac prevents some software, such as startup items, from loading, and it performs a check of your startup disk. Your Mac may take longer to start up because of the check.

Is your data backed up?

How full is the hard drive, and how big is the hard drive?