Scientists think the most likely reason for the dinosaurs becoming extinct is that the Earth was struck by a very large meteorite. This was such a large asteroid that it caused very large amounts of rock dust and ash to be thrown into the atmosphere.
This dust and rock will have shielded the earth from the suns rays causing drastic climate change. The Earth would have got colder, plants would have died due to the lack of sun. It is believed that dinosaurs were unable to evolve and adapt fast enough to these changes and died out.
it wasn’t just dinosaurs that died out during this period around half of all life on the planet died over the same time period.