From memory the closest event would be Helios Airways Flight 522 crash, where passage and crew were incapacitated by hypoxia. A flight attendant who was a certified commercial pilot (but without experience on a 737) managed to resist awake thanks to a portable oxygen supply and enter the cockpit.

He even waved at the chase F-16 next to him but one engine flamed out as soon as he entered and the plane started to descend, crashing just 15 min later.

Maybe if he had had more time (and oxygen) he could have managed to bring the aircraft to an airport, but we will never know.