The gods would lose their power, and as the consequence the wall of Tartarus, the underworld prison, would break.
That's what Zeus (Liam Neeson) said to Perseus (Sam Worthington) about what's happening. He then asked Perseus to help him and the remaining gods to battle this if something bad happened, but Perseus declined the offer. He didn't want to leave his only kid alone.
Finally, it's only Zeus and his other son, Ares (Edgar Ramirez), also Poseidon (Danny Huston) who were left to fix the wall. Zeus also asked Hades (Ralph Fiennes) favour to forget what had happened between him and Hades, and helped them. The unexpected happened. Hades released his minions to attack them, and Ares betrayed Zeus and helped Hades captured him.
