If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
It's horrible that zombies are at your gates! In order to survive, you must build flying castle and leave the danger zone full of zombies.
Try to collect more resources. The more you collect the more quickly you will build the Flying castle and leave the zombie zone. The more boxes you will destroy the more quickly you will be able to buy better weapon. The Flying castle also will help you to kill zombies.
Don't forget to collect and use stars to upgrade your abilities (speed, lives and magnet).