Problem with external monitor and MacBook Air I cannot connect my MacBook Air to an external monitor. But I am able to connect it to a projector, without any difficulty. My computer detects the external monitor but cannot connect to it. The screen is just black


I have tried everything in the System Preferences > Display > Resolution
category. Does not work, but any clue will help!
I have upgraded everything on my Mac, so that it is now OS
X El Capitan. 
I have tried to deconnect and reconnent the cable connecting to the screen. 
Other MacBook Air work with the monitors I want to use, and they all work.
I have tried the sleep-mode to get it to detect the mac/screen.
I have tried changing the VGA cable. 


I have tried almost everything, what am I missing?
Are you connecting using 100Hz for example on a 60 Hz monitor? The monitor will turn black then and appears to be disconnected.
There should be an option to cast it as a different Hz to your monitor.