When you have a laptop or similar computer with more than two displays (laptops have one screen and then one external VGA adaptor, or some other connector in the case of the MAC computers), have a different modes of showing the image.
In the case of the laptops, there is generally a combination of keys that will take you to the following screen (generally Function-F2, or Function-F3)
When you have no image, it is generally because you are on a computer only mode (only one screen).
You can select Duplicate (same image on both screens), or Extend (big desktop), or projector only
Extended mode is the best mode, because you can have your own notes on the laptop, and use what is called presentation mode, that looks like this in powerpoint: