I was playing around with the external display settings (while it was plugged in) to try and get the sizing right. Unfortunately I clicked on 4096x2160.
Now when I plug the external display in, my macbook screen goes black and just shows my desktop icons (no open windows) and the external display shows thin coloured lines. Now I can't change the settings at all because I can't see the window where you change it. Does anyone know how to change or reset the settings without the external display plugged in?