Try pressing Fn + F4 numours times. You do it once it should display on your Monitor, again maybe back to normal and again maybe on both.
Try pressing them over and over and see if you get output.
If not, right click on your desktop and go to settings -> Monitors OR Advanced. One of them. Browse around and see if theres any options where you can click and select both.
When I tried it with mine I couldn't do it either at first. I have to press Fn + F4 (dont know if thats the same on all laptops). I pressed it once and the screen appeared on just the monitor, pressed it again and it appeared just on the laptop but when I pressed it again it was still just on the laptop. Shortly after however it did also appear on the monitor.
I would just try leaving it on each setting for a short time and see if it eventually works.