https://www.scientificamerican.com/article/what-killed-the-dinosaurs1/

Paleontologists have long searched for clues as to what caused the mass extinction of the large dinosaurs. How did hundreds of species vanish so abruptly, leaving us only their fossilized remains as clues that they ever existed? Such a devastating event would have required a sudden onset of dramatic changes in the Earth’s climate.

This event in time is known as the K-Pg boundary, or the Cretaceous-Paleogene boundary, a marker between an era when dinosaurs roamed the Earth and a world more like today. We can trace the Earth’s history to study this time period by delving into deeper and deeper rock layers which each carry an imprint of the events that occurred when those rocks were once at the surface. The thin layer of rock at the K-Pg boundary marks the end of the Mesozoic Era and, along with it, the end of most of the Mesozoic species, including all non-flying dinosaurs.