try writing the page_load event in the following way-
protected void Page_Load(object sender, EventArgs e) {
if(!IsPostBack)
{
string PID1 = HiddenField1.Value.ToString();
if (PID1 != "")
{ DataTable ds = obj.GetPatientDetailsById(PID1); TxtPdFirstName.Text = ds.Rows[0][1].ToString(); TxtPdMiddleName.Text = ds.Rows[0][2].ToString();
}
}
}