The last dinousaurs died approximately 65 million years ago. The cause of their extintion is still unclear, though there are several theories that may explain.
1. A huge metheorite crashing near the Gulf of Mexico, that could have landed with an impact and created a cloud of dust big enough to blockout sunlight for a good amount of time, creating a deadly chain of events, no sunlight, no natural processes, no life.
2. Ash and gas coming out from the Volcanoes.
3. Diseases spread through the dinosaur population.
4. Food chain imbalances.