Do you get any image on your external? Have you moved the mouse both to the left and right of your screen to see if it shows up? If the monitor is set to be to the left of the screen but you have it physically on the right it can make you think it isn't working because you have to go to the left to make it show.
Within your display settings you should see both monitors listed, and you can hit identify, it will put a number on each screen, have you tried that? It will tell you if the screen is seen by the laptop and also whether it is seeing it to the left or to the right (and you can drag it to change which side).