If your still in the game or if it happens again,go to where the outside mysterybox location is and where the street is "lifted off the ground" and you cant get up there(this is also where you can get into town from diner/farm) they usually get stuck there. Not sure whyy guess is a zombie mightve pushed him into a frozen spot). Just lob grenades over and kill it or you could try shooting it while jumping and it should start up the match again
TL:DR lob grenades at the shortcut entrance from diner/farm
GT and PSN: ReCrisis Kick back, relax and enjoy the world around you~!