ubuntu 20.04, second monitor on hdmi port not detected

Check if you have the latest drivers

sudo ubuntu-drivers autoinstall

Then check for your additional drivers. Try to go to

• Software & updates
• Additional Drivers (And there you see your drivers.)
• Change your card driver then press apply (even if it is a good driver),
• Then change again to the latest driver.
• reboot

This resolved my issue, I think because ubuntu did not recognize drivers because of some updates.

Hope it helps.

I have Ubuntu studio 20.04. I upgraded my ASUS motherboard BIOS and one of my two monitors stopped working. I looked at the output of dmesg and it showed this:
[ 50.822389] Lockdown: Xorg: raw io port access is restricted; see man kernel_lockdown.7

Some googling showed that the BIOS UEFI safe boot locks the kernel and won't allow the Nvidia drivers to load. Dual boot won't work either with a safe boot.

I followed these instructions to remove safe boot: How To Disable Secure Boot on an ASUS Motherboard. After rebooting both monitors were working.

Just spent hours trying to fix the same problem. Didn't realize I was using a directional HDMI cable.

Please flip your cables around before going crazy looking for solutions, it may be as easy as that...