When did all dinosaurs die and how?

There were a lot of different kinds of dinosaurs that died off at various times. However, pretty much the final curtain was something called the K-T Event, which marked the end of the Mesozoic Era and the beginning of the Cenozoic Era. It's thought that a large meteorite (perhaps 10-40 km diameter) struck the Earth, resulting in climate changes which most dinosaurs were unable to adapt to.

The K-T Event happened roughly 65 million years ago.