Love the game! One thing that happened to me in horde mode was that it got really intense, I figured out how to survive in the game and it seemed the playing was so intense that the game froze. This is the screen it froze on. I thought I may have hit a pause button or something, but I don't know.
EDIT: Here is a video of gameplay of Horde mode for anyone interested. I think this is a pretty fun game.