One thing i can think of is if you have other programs open like Steam, Origin, Uplay or any other program that has to update regularly. They will usually have a pop-up to let you know about the update and if you have your game in fullscreen it will minimize your game. You could try some common fixes to this kind of stuff including verifying your game via Steam, changing your game to windowed mode and making sure you have all of your graphic drivers installed correctly and making sure that they are not corrupt (that seems to happen a lot to me and it creates a black screen every once and awhile).
Also do you have this problem in any other game(s) and if so do you have those games in fullscreen mode? lastly if you happen to be ingame before you try to fix it and it happens again try to go into your task manager and you should see "Arma 2 OA" and there should be a drop down bar and you should see another "Arma 2 OA" right click on that and click on "bring to front" if that doesn't work try "maximize".
