What event cause the dinosaurs to become extinct?

No one knows for sure, but there is a good chance that the dinosaur extinction came about as a result of a very large meteor strike on Earth. Such a strike would have filled the air around Earth with dust, blocking a lot of sunlight. Without enough sunlight, plants would die back, causing many dinosaurs to starve. (They needed tons and tons of plant material a day to eat.)

With a scarcity of plant-eating dinosaurs, the meat-eating dinosaurs in turn starved to death. Little animals that did not eat much mostly have survived.