Laptop is opening with a 2nd screen set as default

Hi peeps,

have a rather ... complicated question, in terms of how to explain my hassle.
I have the problem that my laptop (Dell XPS12 flip-top Windows 8) which occasionaly starts with a black-screen.
I had this problem a few weeks ago and thought the graphics driver was screwed. I got absolutely NO response and couldn't change anything. As far as I could tell (which was confirmed after a Dell bios check) everything was fine ... I just couldn't see anything!
What I've come to realise is that the laptop is opening with a 2nd screen set as default (is this even possible when not attached?) and not the built-in display. I was travelling when the problems came up before so I didn't think of that but this morning I started to the black screen prob again. After quick contemplation I plugged in my external screen and Viola! the Log-in screen was waiting for me.
How can I change this? Seems to be an occasional problem (after using the ext. screen) but the laptop is useless when it opens this way and I don't always have a second screen with me to resolve the prob ;)
thanx in advance
ps.