Fox reporting it was reported missing late yesterday. Supposed to have 96 hour air supply and has a way to release batteries so it can float to the surface. So if the problem was not one of an implosion, they may have a chance. Otherwise it’s a whole different story.
Failure could be air or air supply - contaminates in the air or a bad CO2-CO scrubbing system. Power, control failure. Hitting the hull - fouling the sub so it can’t rise up.
Not necessarily a hull collapse, but a “failure to be able to rise” could stick them below.