“How dangerous is a collapsed lung?”

The seriousness of a collapsed lung depends on how much of it is collapsed, why, and how fast it happened. So it is therefore impossible to answer that question safely without more information as the answer would be misleading. Suffice it to say that if an individual believes he has collapsed his lung, for any reason, he should promptly report to an emergency room for immediate evaluation.

You can die, it causes reduced blood return to the heart.