This sounds like bug 1872159... Please try editing /etc/default/grub and remove the word "splash". Then run:
sudo update-grub and reboot. Now can you boot with the monitor connected? ** Summary changed: - 20.4 - hybrid-graphics + (If external monitor connected) Hang on boot while showing ubuntu boot screen with animation ** Package changed: xorg (Ubuntu) => plymouth (Ubuntu) ** Tags added: multimonitor ** Tags removed: multimonitor ** Tags added: mulimonitor ** Changed in: plymouth (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Ubuntu-X, which is subscribed to xorg in Ubuntu. https://bugs.launchpad.net/bugs/1878421 Title: (If external monitor connected) Hang on boot while showing ubuntu boot screen with animation To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/plymouth/+bug/1878421/+subscriptions _______________________________________________ Mailing list: https://launchpad.net/~ubuntu-x-swat Post to : ubuntu-x-swat@lists.launchpad.net Unsubscribe : https://launchpad.net/~ubuntu-x-swat More help : https://help.launchpad.net/ListHelp