Did you have enough rest after your last battle? Get up and deal with your enemies again, because zombies are back. Place units on the position and upgrade them when you have enough money.
Sometimes your old enemies return to complete what they wanted to do last time. So you better be on the guard and resist them when they come again. Zombies never retreat and will make to the end. Command elite squad of brave soldiers and make sure you have enough resources to defeat them.