On a large commercial aircraft, if both pilot and co-pilot were to become irreversibly incapacitated simultaneously, the first problem would be gaining access to the cockpit since the cabin door is locked from the inside.
If a passenger was able to employ formidable powers of strength and force entry, remove one of the pilots from their seat, contact the control tower, receive messages from a qualified pilot or instructor and activate the autoland system then it is highly likely that the aircraft would land safely, provided conditions on the ground are relatively stable.
Autoland systems are designed to work in situations where visibility is limited, usually due to adverse weather conditions, but not in strong, gusting winds.
If an unqualified pilot was not able to activate autoland the landing would be more problematic, but still feasible.
In an episode of MythBusters, two presenters with no flying qualifications were able to "land" an aircraft at the controls of a NASA flight simulator when they were talked through the procedure by a pilot.
There is no record of a successful talk-down landing of a large commercial aircraft, but neither has a case arisen where both pilot and co-pilot were taken out of action other than by force.
However, it has happened that a passenger with no flying experience has taken control of a light aircraft when the pilot has been incapacitated and been talked down to a successful landing.
Crucial to this success is the role of a flight instructor relaying instructions from the ground.