Answer:
All plants and animals on earth engage in a process called respiration. Respiration combines oxygen and the food created during photosynthesis to produce usable energy. One of the byproducts of respiration is CO2, this is the opposite of photosynthesis. It is not unusual for plants to stop taking in carbon dioxide to uptake small amounts of oxygen at night.
In the event that plants cease to take up CO2 all together, they will cease to convert sunlight into carbohydrates, and die. The death of autotrophs would lead to a chain reaction of the death of all higher order organisms. Mankind could survive on food stuffs on hand, but only for a very short time. It has been said that any western society, its members are just 3 meals away from revolution.
Social order would be completely lost during the initial food riots. Once the processed and packaged food supply is exhausted, it would be the law of the dying jungle. The strong and well armed would take from the weak and ill prepared. Livestock and game would be slaughtered as the next to the last food source disappeared. The shear volume of death would foul the water spreading pestilence and disease. In one or two years bands of humans would prey on other humans as they degenerate into cannibalism. There could be a few humans in protected bunkers and hideaways, but they too would eventually succumb to starvation or despair as the world died.
The fact that plants stopped converting CO2 into O2 would be of little consequence. Man would die of hunger 1000 years before oxygen was ever an issue.