I don’t know how the Nouveau driver was blacklisted during the update process, especially since I don’t have proprietary drivers installed on my system. I booted into an older kernel and noticed that the Nouveau module was not loading.
After some web searches, I pressed ‘e’ during boot and discovered that the Nouveau driver had been blacklisted. I edited the boot options, removed the blacklist commands, and added nomodset. After booting successfully, I updated the GRUB configuration file in the /etc/default/grub directory to make these changes permanent. As a result, I can now use my external monitor again.