What happens if your lung collapses?

See a doctor, urgent: A collapsed lung (pneumothorax) is due to an air leak from the inside of the lung through the covering of the lung, out into the space between the lung and the ribs. The site of the leak has some damage, which will heal itself later. But the leak may continue, with leaked air filling the rib cage (chest cavity), pushing on the heart and collapsed lung...Causing death. Pneumothorax is an emergency!

Must re-expand it: Depending on the reason and the symptoms, but the first thing is to re-expand the lung, usually by incerting a small tube into the chest cavity. It is not good to have a collapsed lung and can even be life threatening. It is important to determine tha cause of the collapse and treat it. Sometimes the cause can not be determined.

Air in wrong place: Pneumothorax is an accumulation of air outside your lung but inside your chest/ribcage. It is commonly described as a "collapsed lung" because the air in this space displaces your lung and prevents complete expansion of your lung.