Blank screen on MacBook Air when connected to external monitor

My macbook air has a blank screen when I turn it on. I've connected it to an external monitor and when I close the macbook. The external screens comes up with the macHD, options across the top and a restart button on the bottom. When I open the macbook to interact with it the screen goes blank again. Not sure what's wrong or how to fix it.

Original Title: Macbook Air blank screen