When you come across an unresponsive patient, the next step is:

Shouting and shaking is checking for responsiveness: The patient is already described as unresponsive, i.e., not responding to shouts or shakes. The next step in an unresponsive patient is to check for carotid pulsations; if not palpable, consider that the patient is in cardio-respiratory arrest and immediately initiate CPR. CPR is initiated as Circulation -> Airway -> Breathing (CAB), not ABC any longer.