This question has puzzled scientists for a long time. We can never know for sure what killed the dinosaurs because we cannot go back in time to 65 million years ago when they disappeared. One way some scientists, called paleontologists, study events that happened hundreds or millions of years ago is by looking at rocks, dirt, dust or sediment (material covering the ocean floor) and the skeletons or shells of animals and plants that became trapped in this material when they died (fossils). Scientists can date material and fossils from the time of the dinosaurs to within a few thousand years but no closer.

In 1980 a famous scientists named Luis Alvarez discovered that when the dinosaurs first began to disappear, rocks and fossils from all around the planet had a thick layer of iridum dust. Iridum is a metal, like iron or aluminum, and is present in large amounts on asteroids. From this finding, Alvarez guessed that a huge asteroid hit the earth 65 million years ago and caused the dinosaurs to go extinct. In the 1990's, paleontologists found the crater from this asteroid on the Yucatan peninsula in Mexico, and found rock fragments from this impact had been blown as far away as Nebraska! Judging from the amount of iridum dust and the size of the crater, Alvarez and other scientists guessed that huge amounts of dust and ash from the explosion were thrown into the sky and calculated that this blanket of soot would have blocked out the sun for many years. Most animals, including humans, depend on plants for their food, either directly (by eating plants) or indirectly (by eating animals that eat plants). If the sun disappeared for even a year there would be no plant growth, and eventually there would be nothing to eat. Alvarez also guessed that without the sun, the earth would have become very cold, and as most people believe that dinosaurs are like reptiles and cannot regulate their temperature, many of them would have died from the cold. Lastly, the asteroid crater was found in a certain kind of rock, which when vaporized would have caused acid rain to fall for years. This acid would have poisoned drinking water and killed the dinosaur's food, if not the dinosaurs themselves.

Many scientists disagree with Alvarez. The biggest problem with his theory is that dinosaurs had been going extinct long before the asteroid impact occurred. What caused the earlier extinctions? Also, why did some types of animals (especially mammals) survive the extinction when so many others did not? Many paleontologists argue that the dinosaurs went extinct gradually, over a long period of time, for a lot of different reasons, while Alvarez and his supporters believe they went extinct suddenly, after the asteroid impact. The fact remains that the asteroid crater exists, so there must have been an impact. Was it the main cause of the extinctions that occurred 65 million years ago, or one of several? The most popular alternative theories include (1) increased volcanism occuring over millions of years, and starting before 65 million year ago, which caused gradual changes in climate and (2) the evolution of flowering plants, which the dinosaurs may not have been able to eat and which could have taken up so much CO₂ from the atmosphere that temperatures declined gradually.

Some good web sites are on this subject are:
http://rainbow.ldeo.columbia.edu/courses/v1001/23.html,
http://www.execpc.com/~maas/extinction/index.html,
http://www.ucmp.berkeley.edu/diapsids/extinctheory.html

There are many thoughts of why the dinosaurs are extinct.Scientists have found evidence that a very big, catastrophic event happened about 66.4 million years ago that very quickly changed conditions in the biosphere, the part of the Earth where life exists. The biosphere includes the land surface, the lower atmosphere and parts of the ocean. One major thought for this catastrophe was that a volcano erupted that was many times more powerful than Mount St. Helens, Pinatubo, or even Krakatoa (all major volcanic eruptions that have occurred in recent history). An amusing website for information on volcanoes is http://volcano.und.nodak.edu/vwdocs/ .
A more commonly accepted theory for the extinction of the dinosaurs is that a very large meteorite hit the Earth (in fact scientists think they have found the crater near the Yucatan Peninsula in the Gulf of Mexico). Unfortunately I can't find any web sites on this but the name of the impact crater is Chixulub. What do you think would happen if a really big meteorite hit the Earth? Scientists agree that the force of the impact alone wouldn't kill all the dinosaurs on the planet (although the ones directly under the meteorite wouldn't be very happy). If the meteorite didn't directly cause all those extinctions, what did? I think you're on the right track with your questions about their food. Think also about how the world's climate might change if you threw a lot of dirt and dust into the atmosphere (there are deposits of dust with extraterrestrial material found all over the world from this period) and realize that the dinosaurs were reptiles and cold blooded.

No one knows exactly what caused the dinosaurs to go extinct. I think the most popular theory is that a big meteor hit the Earth and the aftermath caused the dinosaurs to die out. What do you think the effects of a large meteor strike would be? Why would the big dinosaurs die but not small mammals and other animals? Keep in mind that "meteor impact" theory is only one of several possibilities.

The dinosaurs and along with them about 75% of all species existing at the time became extinct 65 million years ago when an asteroid about 10 to 20 kilometers in diameter collide with earth in what is now Yucatan !!!!!
The effect of this impact was TREMENDOUS--so much dust went into the atmosphere that it was totally dark for SEVERAL MONTHS. This killed most plants. Other things happened also. Fires were burning all over the planet due to the tremendous SHOCK wave set up as the asteroid passed through the earth's atmosphere.
I could go on and on about this event. but perhaps you would enjoy learning about it on your own. The event is called the KT impact event. there are manty articles on it. do a computer search and look up the name LUIS ALVAREZ, the man who discovered most of this.

I did a search on "dinosaurs" on the web, and found a good site with a lot of links, including links on their extinction:
http://www.ucmp.berkeley.edu/diapsids/dinolinks.html#dinosites
However, your best bet is probably a library - any book on dinosaurs should discuss their extinction.