Try something like following-
select patient_mstr.Sr_no,patient_mstr.P_name,patient_mstr.M_name,patient_mstr.L_name,old_patient.E_date,patient_mstr.Address,old_patient.Pain,old_patient.Total_charge,old_patient.Total_paid
from patient_mstr
left join old_patient on patient_mstr.Contact_no=old_patient.Contact_no
where old_patient.Contact_no=@Contact_no
Please let me know, if it doesn't help :)