This was happening to me too. I tried everything AnchorTheMean suggested, but the only thing that would work was starting it in windowed mode. You can't change it back ingame, so I had to download another program that runs windowed games borderless to get it full screen again, but even that didn't fully work. It started with Windows10 Anniversary Ed. like OP and this is my only fix so far. What has Microsoft done with this update?