A collapsed lung occurs when air escapes from the lung and fills up the space outside of the lung, inside the chest. It may be caused by a gunshot or knife wound to the chest, rib fracture, or certain medical procedures. In some cases, a collapsed lung occurs without any cause. You will most likely survive if treated quickly.
I know of someone who had lung cancer and had one lung removed. He was such a keen hill walker that he carried on afterwards with very little problem at all, in fact over time his one lung grew to 75% of the capacity of two separate lungs.