How can I use a VGA cable as an input?

Having trouble with a VGA cable..?

• So I've got a laptop with a very broken screen, and a perfectly working separate screen. I've got what I'm 99% sure is a VGA cable. I'm trying to use the VGA cable to plug the laptop into the screen so that I can use the laptop as though it were a desktop computer. However, when I plug the VGA cable into both the laptop and the screen, the screen says "No input signal" and goes into power saving sleep mode. It's definitely recognising that something's been plugged in, because when the cable's not plugged in it says, on the screen it says "Cable unplugged" (and then goes into power saving mode). I've done this before with another broken netbook and it worked fine, so it's probably the laptop, although I'm certainly no expert. I've (hopefully) made no silly mistakes- the screen's plugged in and the laptop's plugged in and fully charged. They are both turned on. The cable's definitely fully plugged in. If you guys could offer any help at all then that's make my year! I hate having to borrow my friends' laptops! Thanks!

• Answer:

  Look along your F keys for one with a symbol that looks a bit like a square with vertical lines either side, or a symbol like two squares side by side. This is the key that "toggles" the video output on and off. You often have to press this simultaneously with the Fn key to make it work. Laptops usually have the facility to turn the external video output OFF and ON to save the battery.