Dinosaurs lived on earth around 65 million years ago. They had a number of subgroups and were found in different sizes. All of them became extint during the Cretaceous–Tertiary extinction event. This was mass extinction and resulted not only in extinction of dinosaurs but also some species like ammonites, mosasaurs, Pterosaurs, herbivorous turtles and crocodiles etc. There have been various studies on this subject and no final conclusion has been adopted for their extinction. However, most of the researchers point towards the fact that this extinction was due to climate changes that resulted in lower temperatures, lower CO2 levels, lower O2 levels etc and thus resulted in extinction of many species including dinosaurs. For details see the link below:
Dinosaurs