You don't say if this is a laptop or a desktop. I am going to assume it is a laptop and tell you that one of the Function Keys (F7 or so) will turn off your laptop's monitor, such that you can use an external monitor, for giving demonstrations,using Visio or another type of application that requires a larger screen than your laptop's screen.
So, it is a triple toggle type switch, such that hitting it once, turns off the internal screen. Hitting it twice turns it off while the external screen is working, and hitting it a third time, allows the internal screen to work while the external screen works, too. That way, you can see your external application working, while you're able to focus on the application on your laptop itself.
See if this is the case, that you have it toggled to turn off the laptop internal screen.
-a.