Reset external display settings

Hi there,

I'm trying to connect an external display to my MacBook Air. When plugged in through the dongle, the display is registered and appears in the display settings, however the monitor is just black. Whilst trying to fix this I enabled the "use this monitor as main display" checkbox, and now whenever I plug the monitor in, my mac's screen is unusable and the monitor stays black. Is there a way I can clear the display settings cache so that this isn't used as the main display when I plug it in? I already tried deleting

/Users/username/Library/Preferences/ByHost/windowserver(UUID).plist

... but this made no difference. Any help would be greatly appreciated.