The most important thing to remember is that there are many theories that are present that attempt to explain the extinction of dinosaurs but there are no absolutes on what actually caused them to disappear. All we know is that there was some major catastrophe that caused their extinction.
One of the theories is the asteroid theory. This theory is very popular and states that a very large asteroid plummeted to Earth. When the asteroid hit, there was so much dust and debris that it blocked out the sun. Because of this, many living things died.
Another theory is that there was a massive volcanic eruption. When volcanoes erupt there is a great deal of ash thrown into the air. This would make living conditions impossible because of suffocation.
There are several other theories regarding the mass extinction of dinosaurs but these are two of the most popular.