We don't actually know that. And part of the reason is "dinosaurs" is such a broad term, encompassing so many species of many millions of years. I would think it's mostly likely that some dinosaurs were cold-blooded and some warm-blooded.

I have also attended a talk that suggested that some dinosaurs almost certainly didn't fit the warm-blooded or cold-blooded model, and that despite mechanically being cold-blooded, their large size alone kept them warm. So in many ways those ones were practically warm-blooded.