I agree with Cool about auscultation being the first step.
But the second step should be to rule out equipment failure and switch the patient's circut, to a wall mounted AMBU bag for example, since the failure to ventilate could be due to a malfunctioning valve in your ventilator circuit.
Intubation should be simultaneously planned, but perhaps without paralysis as this patient's airway may have collapsed due to an intrathoracic mass and will be negative pressure ventilation dependent.