If you have an older computer you should first connect the VGA fly-lead to your machine (with it powered off), then switch on the data projector and select the correct source on the control panel (PC or data - check the wallmounted AV instruction sheets), then lastly power up your laptop.
Most modern laptops will simply project when the cable is connected. Many rooms will also now have an HDMI connector.
Please also see The picture appears on my laptop screen, but not on the projectory screen?
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This is question number 1392, which appears in the following categories: