I doubt I would ask a passenger if they knew how to fly a plane tbh because who knows what weirdos may say yes, and then of course there is the terrorist threat.
Firstly, I would probably be getting another crew member to bring me oxygen etc and help me administer 1st aid to the incapacitated pilots. If the aircraft was flying itself and was stable, that would be my first priority.
Then it would be to contact the ground as quickly as possible to alert them as to what has gone on and to get someone to talk us down. Most modern aircraft can now land themselves anyway.