Dust thrown up by the impact would have spread out covering the entire world in darkness. Temperatures would have dropped precipitously. Plants would have failed to receive enough sunlight to allow photosynthesis to continue. Conditions in the oceans would not be much better as massive acid rain buildup poisoned the water and destroyed shell-bearing creatures, disrupting the entire food chain. This included the dinosaurs which had dominated the landscape for over 160 million years.
Some scientists remain unconvinced by the evidence, however.
The following web pages discuss this mass extinction event as well as others before and since. Asteroids of Death by E.S. Matalka discusses the asteroid impact hypothesis for the extinction of the dinosaurs. Matalka is critical of this idea. He suggests David Archibald's marine regression hypothesis is more likely. Carriers of Extinction by Carl Zimmer suggests that the megafaunal extinctions at the end of the last ice age were caused by pathogens carried by migrating humans.