Most laptops have 3 monitor settings. Their screen only, output through their VGA/HDMI only, or output to both simultaneously. When you toggle between them you may not know where you are starting in that cycle, so be sure to toggle at least 3 times and be sure to pause for at least 3 seconds between each toggle. Also make sure the cable you are using is good, that one kills many times.
Not all laptops are function F7, so make sure the function key you are using has a picture that says CRT/LCD or a picture of a monitor...
And last but not least a lot of fancier laptops have a monitor out AND and monitor INPUT, so make sure you are connected to the monitor output of the laptop if it happens to have multiple VGA/HDMI connections.
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