A collapsed lung which is known medically as pneumothorax can sometimes heal on it's own as long as the collapsed lung is not too severe.
If the collapsed lung is more severe then it requires medical attention and is an emergency situation that requires a hospital visit to keep the person alive.
The medical treatment for a collapsed lung or pneumothorax usually involves the hospital inserting a needle or chest tube between the ribs to remove the excess air.
Then the collapsed lung returns to normal and within a day or so you're usually back up and walking around but you should still take it easy until you feel much better.
Many people do live quite a long time even after suffering from a collapsed lung and it's always best to get medical treatment for the collapsed lung even if you think it's not very severe.
Because if the collapsed lung is very severe and really collapsed then it prevents the person from breathing which can be fatal if not treated in time.