The monitor is still connected. I didn't change anything, one day it was working and the next it wasn't.
Then I'll repeat my question again. I don't understand why you can't simply go to the monitor you placed the window, move it back to where it was, and re-dock it. If the window has been created, it must be either on one monitor, or the other.
Maybe you are just ASSUMING the window is "gone because you undocked it", but maybe this is not what's really happening, and GSX is not even creating the window, because of an entirely different problem with it, that by pure chance happened after you undocked the window, so you think it has something to do with it, when instead it's simply a problem of GSX not creating the window.
BTW, I think you shouldn't use the Spacebar to call GSX, because it's a key reserved by FSX for the camera view. If you can't use the default CTRL+F12 keys, try to use something specific that FSX will never use, like TAB+F12.