There is no single reason that can be given for the disappearance of dinosaurs from the earth. One theory is that a huge meteorite struck the earth, turning it into a cold dark place and killing off all creature that needed warmth and light for survival.
It is also possible that volcanoes all around the earth erupted at around the same time, sending huge quantities of lava dust and deadly gases into the air, to wipe out entire species. The climate would have become freezing cold with the advent of Ice Age, and dinosaurs could not have been able to adjust to the change , since they needed the heat of sun to survive.
