If your lung collapses is that fatal?

It can be.: If your lung collapses from a pneumothorax, or air in the lung, it can be fatal unless you get emergency treatment, like a needle between the ribs on the affected side, and then vacuum tube on that side to make sure the lung gets filled with air. Otherwise death can occur from heart failure due to lack of filling from the pneumothorax. If due to broken ribs, it can also be fatal.

Potentially: Sometimes, if the pneumothorax creates enough pressure, it can compress the right side of the heart. This can decreases the blood return to the heart. Without adequate blood return, your heart does not have enough blood to pump out to your body. This is called a tension pneumothorax. This is potentially rapidly fatal.