do you know the default resolution of your external monitor?
if it's recognized, go to the screen resolution settings (right click the desktop > properties > settings)
you should be able to see that you have 2 monitors connected, number 1 primary, this is your laptop screen, number 2 should be your external.
what you want to do is click that number 2 box and then set the resolution to the one your external monitor supports.
if there's still no signal either your VGA ports are malfunctioning or your VGA cable is not working