How to force Ubuntu to detect external display?

Solution 1:

Actually, you don't need to log out. Simply going to a VC with ctrl-alt-F1, restarting x with sudo service sddm restart and and going back to your graphical interface with ctrl-alt-F7 (or F2) should do it.

This way, you don't lose all your windows...

Solution 2:

To enable all outputs in their default mode, run:

$ xrandr --auto 

For more information, see:

https://xorg-team.pages.debian.net/xorg/howto/use-xrandr.html

Solution 3:

I just encountered this issue with my Dell Latitude E5550 using a port extender and two external monitors.

My problems began when I had a power failure. For silly reasons, I could only plug in one of my two monitors, so I opened up the screen display interface and de-activated the screen that didn't have power.

Awesome!

Once the power came back on the process to turn my screen back on was unintuitive. I couldn't see it in the screen display interface even after rebooting or redocking my laptop.

So I tried $ xrandr --auto and got Ubuntu to extend to "one" screen but duplicate to both of my externals (they are two of the same model). Amusing but not useful.

It was only when I clicked on "1 Built-in display" like in the screenshot below that I was able to select my "3rd" screen and turn it on again.

The Screen Display Interface

Doing this flipped the two screens around for some reason and I had to rearrange them but that was easily fixed!

Solution 4:

I also have Dell Latitude E5550, I have one onitor via hdmi, and one via usb-c. Then one via usb-c is not detected sometimes, and for me what it works, is unplug/plug the hdmi port in the monitor, while the rest is connected.

Solution 5:

Working solution: Disable secure boot in the bios/efi menu.

Hello Everyone, I am coming from a TongFang GK5CN6Z and I had this issue. I noticed it was an issue after having ran the latest updates on Ubuntu. Turns out I had to disable secure boot in order for it to work.