Whenever I press alt+tab to go to my desktop or other opened program, when I tried to alt+tab again to DAYZ, the game wouldn't return, as the monitor stayed black and then an "Not optimum mode" message appeared from my monitor, probably beacause it couldn't find the resolution to return to the game.
The "solution" was do alt+enter or any command that would get DAYZ to windowed mode, then the image returned and I could go back to fullscreen mode without any problem again