I am running Linux Mint 17 Xfce on an HP Elitebook 840 G1 laptop. Up until just now I was using an external monitor, displaying identically the same thing as the built-in laptop display, connected via the VGA port. This worked absolutely fine and as expected.
However, I have accidentally unchecked a tick-box in the
System Settings -> Display options, which said something like "use this display". Now the external monitor has vanished from the system settings, seemingly never to return. Of course I have tried restarting the computer and monitor, unplugging the VGA cord etc. Even worse,
xrandr now cannot detect a VGA output on my laptop at all! Anyone have a clue what I've done here?
I should add that the display works while the computer is booting up, and also works fine at the login screen. Only once I have tapped in my password does the output to the monitor suddenly stop. I also have Windows installed on another hard drive on the same machine, and the external monitor works fine for that. So there's clearly nothing wrong with the hardware.