Hello,
My name is Mike, I’m a Technical Analyst located at Dell corporate headquarters in Round Rock, TX. I'm part of an internet outreach team developed to interact with the online community regarding technical questions and issues that customers face with Dell products. I read your question and wanted to chime in.
You switch the primary display by holding down the <Fn> key and tapping on <F8>. It has 3 settings, internal LCD/Internal and external LCD/external LCD. It may take a second for the settings to kick in so just hit the Fn+F8 key once and wait for a few seconds, if nothing happens try it again.
It may also be something as simple as a loose cable. You can reseat the connection by looking at the manual
here. It has pictures and text so you can see what you're looking at for before you try anything. I hope this is helpful.
Mike
Dell customer advocate