I upgraded to ubuntu 17.10 yesterday, and now my dual monitor setup is not working. The screen is detected, and I can move my cursor over to the second screen, but it is completely black save for the cursor.
The computer is a Dell Optiplex 9020
EDIT: I tried to deactivate and then reactivate the secondary monitor. I'm not stuck in "Single Display" mode. Attempting to change to "Join Displays" causes me to get logged out, and when I attempt to log back in, I'm back in "Single Display"
21 Answer
Open the Activities overview and start typing Displays.
Click on Displays to open the panel.
Click on the monitor that you would like to activate or deactivate, then in the list of monitor configuration select Turn Off to deactivate the monitor, or one of Primary, Secondary, or Mirror to activate it. To rotate the orientation of the monitor, use the buttons with the arrows until the preview is correct.
When you are happy with your settings, click Apply and then click Keep Changes.
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