Please help with this problem i've had for a YEAR! Cant get both external monitor and laptop screen to work!

I have a Dell vostro 1510 laptop which is running windows 7. When my laptop is on I plug in the external monitor via a VGA cable nothing happens. The laptop doesnt recognized that anything has happened. 

Then when I restart the computer WITH the VGA cable plugged in which is connected to the external monitor, only the external monitor comes on and the laptop screen remains black.

I do have a function key on my laptop that has "CRT/LCD" on it and when i press it the screen goes black for a second and then comes back again. Still no dual display.

Any help please!!