I connected a monitor to my laptop and I figured out how to turn off my laptop scrn, change back?

I got a monitor and wireless keyboard to hook up to my computer and I did all that a year or so ago. I was able to make it so when my laptop was hooked up to the monitor the laptop screen would go black. The problem is now when I do a presentation and I need to hook it up to an external monitor I can't see what is on the screen, so I need to change it back does anyone know how. I know it can be done I have done it before.