What do you do if you find a neighbor unresponsive by their house?

By saying 'by their house' implies the casualty is outside their house? So entry into a locked house is not required. Telephone the police if the casualty is inside a locked house. If outside the house, telephone for medical assistance, and while waiting the arrival of an ambulance, carry out chest compressions until medical help arrives. Mouth to mouth is not advocated so much nowadays, but chest compression is.