If backup systems conk out
The plane is also equipped with battery systems, reserved for when all other systems fail. But since we're going down through worst-case scenarios, let's say those are also kaput. All is not lost! Though there is not a recorded instance of this in recent commercial aviation history, pilots train for this scenario in simulators all the time.
Commercial aircraft aren't designed to be gliders, but in an emergency they can be. In planes with hydraulics, pilots can control the rudder, stabilizer, and landing gear using only hydraulic systems. Even if one of these fails, most planes are equipped with multiple, redundant hydraulic systems (typically three) so that if one fails, the others can take over.
It's not so different in a plane with a fly-by-wire system. Those also have a mechanical backup that allows pilots to operate the plane if the power goes down