First - Bind the dropdown on page_load event.
SqlCommand cmd = default(SqlCommand);
SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["Callregister_connectionstring"].ConnectionString);
con.Open();
cmd = new SqlCommand("Select * from Call_Reg where Complaint_no='" + ddlupCompl.SelectedItem.ToString() + "'", con);
SqlDataReader reader = cmd.ExecuteReader();
int i=0;
while (reader.Read())
{
txtRegdt.Text = Convert.ToString(reader["Reg_dt"]);
txtComplnno.Text = Convert.ToString(reader["Complaint_no"]);
txtFname.Text = Convert.ToString(reader["Cust_FName"]);
txtLname.Text = Convert.ToString(reader["Cust_LName"]);
txtPh.Text = Convert.ToString(reader["ph_no"]);
txtMob.Text = Convert.ToString(reader["mob_no"]);
txtemail.Text = Convert.ToString(reader["email"]);
txtadd.Text = Convert.ToString(reader["address"]);
txtcity.Text = Convert.ToString(reader["city"]);
txtdist.Text = Convert.ToString(reader["dist"]);
ddlstat.SelectedIndex =
ddlstat.Items.IndexOf(ddlstat.Items.FindByValue(Convert.ToString(reader["state"])))
i++;
}
con.close();