A couple months ago I was in the Seattle public library and overheard a pierced, tatooed, and quite smelly young man telling someone he was waiting for this F-d up civilization to collapse and hoping it would happen soon. The two most likely causes of such a collapse would be an asteroid or comet impact that would throw debris into the atmosphere and stop food production for several years, or a plague. A big impact or an engineered pathogen would be the extreme in this scenario and would not simply take us back to the stone age– it would render the human race extinct.
All the disenchanted Americans who look forward to surviving the collapse of the present world order might want to consider the less fortunate areas of this planet where there is no such rule of law or any agricultural or industrial infrastructure. North Korea has gone through the classic collapse cycle during recent bad winters and the government had to repeatedly deal with widespread cannibalism. It is one of those most perfect warnings where nothing could be more crystal clear to a race of intelligent and technologically advanced beings. And we ignore it.
Turn the sunlight off for a couple years in a row and everything we know would end because everything we eat would end. Think about it the next time you watch an episode of the Walking Dead or watch a movie like The Road. Not world war Z; world war C.