A very simple question with a very complex answer. Various hypotheses have been postulated, the only almost generally agreed upon thing is that an impact from space near Chixclub, Mexico 65 million years ago dealt the final blow. There is evidence however to suggest that the great reptiles had been in decline for some time, possibly due to disease, climate change, or simply becoming too large to mate effectively. After the initial devastating impact, the frictional heat of reentry generated by the molten ejecta blasted into orbit may have caused a global firestorm, destroying many survivors. The remainder would have starved during the long impact winter which followed, in which sunlight was blocked by dust in the atmosphere. https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcQ9Q5OjyqCz_77JWEjhr-b5RiP-qgOhhFTkFuW4rqVfx8qvWr27