That’s a worrying situation. Here is what typically happens in such situations:
Usually, authorities and the airline will conduct investigations into why and how such incidents happen. Both pilots could face serious disciplinary actions and possible legal consequences as falling asleep during a flight goes against the expected duties of a pilot. Additionally, Airbus, Boeing, and other aircraft manufacturers design their planes with multiple safety features to prevent or correct deviations from planned flight paths.
Further, airplane systems include autopilots that, when properly set before the pilots fell asleep, should maintain the flight path even if the pilots become incapacitated. If a serious incident occurs, the Air Traffic Control could attempt to communicate with passengers or flight attendants, and situations like these also highlight the relevance of the policy in some airlines that stipulate pilots take turns sleeping on long-haul flights while the other monitors the aircraft systems and communicates with ATC.
The investigation may also focus on what could have led both pilots to fall asleep simultaneously – possibilities include fatigue, health conditions, or even potential exposure to carbon monoxide or other toxins.
Safety is paramount in aviation. So, this incident will be taken very seriously, and actions will be taken to prevent similar instances in the future, improve training, and enhance the checks on pilot alertness before, during, and after flights.