A large asteroid from space crashing to the earth, producing a large amount of debris in the atmosphere.
The high amount of debris created dirty "clouds" which blocked out the sunlight and reduced the amount of sunlight reaching the earth.
Because of the reduced sunlight, plants began to die off. These plants included those which served as the dinosaurs food. The dinosaurs also began to die off due to the lack of food.
Over time scientists have had several theories regarding the disappearance of Dinosaurs. As I mentioned this is the current idea.