My iMac has seized up completely after auto install of latest version of Big Sur. After several restarts I managed to force quit open apps, but the iMac has still not booted up. The Dock has reappeared, but the menu bar has not.

Boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and reboot normally and check to see if the problem persists.

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 the problem still persists make sure you have a full and recent backup of your user data and Boot into the Recovery volume (boot with the Command + R keys held down) and reinstall the system.

Personally, I don't have the system updates set to auto install. If there's a problem that could interfere with my project and getting things done in a timely matter. I wait till any projects are completed and check these forums to see if the latest update went smoothly for the most part before running them.

When it says that your computer restarted because of a problem, there should be a checkbox for whether or not to reopen your windows. Force a restart again by holding the power button on the back and then press it again to turn it back on. Then uncheck that box once it reboots. If this does not help, restart again, and this time hold the shift key while booting. This will make it run in Safe Mode. See if the problem still occurs then.