Did anything come on as it was starting before Windows shows or is it too quick to show anything?
Look for the key that toggles the laptop screen - it will have the symbol of 2 screens or 2 computers on it - you may also need to press the Fn key.
This is to toggle between laptop screen, laptop screen and external screen or external screen only - so you may need to do it more than once.
eg. if its on key F2 then press and hold Fn then press F2 let go, press F2 again if, let go etc.