Laptop screen goes black when VGA is connected

[Beaugrand]

Why, after years of use, does the screen go black on laptop when VGA cord is connected even though the TV screen works just like it was the computer. Laptop Dell Latitude E6400 W7

 

[Phillip Corcoran]

Look in your display settings. It may be set up to turn off the laptop screen when external monitor is connected, so you need to change or untick that.