IF this turns out to be a drama with two unconscious pilots in a de-pressurized cockpit, that would become a very important question.
It's an important question but it was sort of covered earlier .... If there's a decompression and the aircraft
doesn't start descending fairly promptly then it's probably reasonable that the Cabin Crew take some action and some are trained to do so
OTOH if there there's a decompression and the Flight Crew
do start an emergency descent then one of the last things they then need is a Cabin Crew member trying to access the flight deck or trying to contact them via interphone.....it's nothing personal, but safely managing an Emergency descent can rapidly fill up the "capacity bucket".
Of course if you still want to go through the "what ifs" you then have to ask yourself - in the worse case if a cabin crew member somehow does gain access to the flight deck and finds that the aircraft is in an emergency descent with all pilots incapacitated what would they (the cabin crew member) do in the time available??