There are many theories of how the dinosaurs became extinct. The most widely accepted theory is that an immensly huge meteor crashed into the earth, causing the skies to darken for months. The temperatures plummeted, and the environment that dinosaurs were used to was wiped out. They died either directly from the meteor or of disease, hunger, and other causes.
Another theory is that they never went extinct, only evolved into the birds and reptiles that we know today. While evolution is supported, dinosaurs must have also died out, leaving their cousins the birds and reptiles.
There are other theories of extinction, but these two are the more popular of the theories.