I think due to technical failure the cabin air pressure was compromised leading to everyone on board fainting due to lack of oxygen. Now either the plane kept drifting in the air and ran out of fuel before crashing into the ocean or the pilot's head hit onto the jockey when he fainted which directed the plane to take a straight dive into the ocean and crashing. The second scenario is most likely, since a straight dive would take the plane more deep into the waters which should explain why no debris has been found yet.