My lung collapsed on its own once. What are the chances it will collapse again?

Depends. First recurrence is most common between 6 months and 3 years. Published 5yr first recurrence rates for primary spontaneous pneumothorax are around 20-30+%. For secondary spontaneous pneumothorax, around 40+%.
Lung collpase. Probably you have pulmonary blebs. If this is true then its more likely that it will collapse in the future.