The debate between whether dinosaurs were hot or cold blooded is still very controversial. Although they're ancestors were cold-blooded reptiles, scientists argue that most dinosaurs were active, warm-blooded creatures. Some lived in very cool climates, and wouldn't have been able to survive if they were cold-blooded. A widely accepted reasoning is that different dinosaurs have different metabolisms; dinosaurs of different sizes may have different heat strategies, depending on their behavioral activity. Research has shown two different examples: Scipionyx's liver and colon are almost identical to those of crocodiles today, suggesting that it might be cold-blooded. Another example is that of Thescelosuarus; a specimen with a rare fossilized heart shows that it has 4 chambers and a single aorta, uncommon in reptiles but found in warm-blooded animals.
Some dinosaurs also have heat-regulating devices, such as a sail or plates that might have been used to heat them.
It is still a objective topic that has and will be argued for a long time.