If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
The zombies are rising from the graveyard. They are coming out from the ground and attacking people. Pull out your gun and kill them all. Do not kill your friends who are in the graveyard helping killing those zombies.
Use your weapon to kill all crazy zombies and Boss. There are 12 levels,4 boss and has lots of zombies