One day, gold suddenly begins to rain from the sky! It fills the streets, covers pathways, and collects everywhere in great abundance! How do people respond? Obviously, they would rush to grab as much as possible, right? What effects on society would this have? If an extremely valuable substance becomes readily available to everyone, does it retain its value? Would it really solve hunger and homelessness issues? Or would it become more of nuisance?
Alright, so this is a rather contrived situation. Afterall, there's only so much you can do with gold rain. So, consider this instead. The point here is not to imagine gold rain, but to imagine a world-shaking event that would alter reality in an interesting way.
Maybe the oceans dried up, leaving caverns and mountains in their wake.
Maybe there's a famine all over the planet but then it's discovered that the moon is made of cheese after all!
Be creative and come up with something unique that changes the dynamic of your environment in a substantial way.