The truth is, no one really knows how dinosaurs became extinct. We know that it is not normal for a whole group of animals to die out at once, and there are lots and lots of ideas about why this happened. A lot of scientists think that there was a big meteorite that smashed into Earth and either killed out the dinosaurs all at once or caused their extinction to happen.
We know about meteorites smashing into other planets, because we can see the craters they made. In fact, in 1994, we saw meteorites as big as Earth hitting Jupiter!
A meteorite crashing into Earth could have caused all sorts of changes on Earth, including changes in sea level, changes in the Earth's orbit, and even changes in weather patterns! All of these changes happening at once could have been too much for dinosaurs to handle.