Laptop monitor is not working

Hello,

My laptop monitor is not working after i plugged into an external monitor. It was still working for the first few times, like both my laptop monitor and the external monitor were both displaying after i plugged the VGA. But after a few times, when i turned my laptop on (seperate from the external monitor), i know it is working but theres no light for the monitor but the monitor is still displaying something but no light (i dont know if you guys understand what i mean). After i plugged the VGA again, both my laptop and external monitor is working again.
So what can i do to turn the light on on my monitor if im not with my external monitor? ( I tried to reboot my laptop when this happened but it is not working everytime)

P.S. my laptop is samsung and my external monitor is asus

Thank you!!!