So I was testing my laptop on my external scree and managed to cycle through the modes until I hit the one which displayed only on the external monitor.

This just output a green screen and nothing else. Now, whenever I connect the laptop I can't use the fn+f3 button to swap between screen modes. It appears that it's stuck on this output.

Also to make things worse, the laptop seems to remember the setting, so now whenever I plugin the display all I ever get is a green screen. I've tried rebooting the laptop, but I think the only way now is to reset the laptop somehow so it doesn't know that it's been connected to the external screen and I can use 'span' or 'mirror' mode instead of 'external only' mode, which clearly doesn't work.

Update

I've tried swapping to terminal using CTRL+ALT+F1 but the external display goes to sleep.

I've tried using microDisplayPort -> HDMI -> DVI to the monitor with the same outcome.

It works fine on my TV using HDMI, but I haven't tried cycling the modes.

I got it working on the TV, and then moved the laptop to the monitor hoping it would work, but I had the same outcome.

I believe there is a setting somewhere in the system which is remembering the last used configuration for the specific monitor, and I would like to reset that, so that I can at least use my screen in span mode.