pulmonary embolism. It's where a clot (usually from your legs) travels to an artery in the lungs causing blockage. If found quickly, it can be treated with blood thinners, but most don't know they have it. Usually, pulmonary embolism is caused by DVT (deep vein thrombosis). I don't think it is common in young adults, but it is a cause of sudden death in healthy, young adults.
An old high school classmate died in his sleep a couple of months ago due to pulmonary embolism. He was a healthy, active 27 year old.