Second monitor suddenly can't be detected (Ubuntu 20.04.1 LTS)
I had a similar issue like this. I downloaded and selected the proprietary drivers I needed in software&updates->additional drivers, and did a couple of restarts. You can check which driver you need with:
ubuntu-drivers devices
Select the right one and restart. It should resolve itself. If that doesn't work try this:
sudo apt-get purge 'nvidia*'
sudo add-apt-repository ppa:graphics-drivers
sudo apt-get update
sudo ubuntu-drivers autoinstall
Uninstalling intel drivers was what actually did the trick for me:
sudo apt-get purge xserver-xorg-video-intel