Q: Headless Mac Mini shows a black screen

Hello,

I have a Mac mini (late 2009) running the latest version of OS X, and I use it as a headless server. Almost everyday I use Screen Share (or Chrome Remote Desktop by Google if I'm using my mobile devices) to manage it and now I noticed when I connect a monitor (via VGA), my Mac doesn't recognise it and it doesn't show anything. I think it has something to do with Screen Share/Chrome Remote Desktop. Do you know what do I have to do in order to solve it? If I restart it it will find the monitor.

Thank you for your time!