Installing Big Sur

See if you can boot into Safe Mode by booting with the Shift key held down.  If successful reboot normally.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

If that fails boot into the Recovery volume (boot with the Command + R keys held down) and reinstall the system.

Force quit by holding down the Power key until it shuts down. Then reboot and it should start up where it left off in the installation process.