If your team loses against another team, you replay the level again against different people. If you're in campaign mode that is.
On campaign once we made it almost all the way through 'Barns' and we got past opening the gates to the stadium, and i was concerned because we had't fought a tank yet. So were chopping our way into the safe room (We're about 20 feet from it) and theres wave after wave of zombies swarming us, and we were stuck... So I sprinted towards the safe room and make it in while my team mates are still stuck slashing their way through zombies. I close the door thining i'm safe, then i turn around and surprise surprise... theres a damn tank... T_T
Off topic XD