how to display the respective states in a text box when a dropdownlist selects a country

ok thanks i will try

protected void newsubgrpddl_SelectedIndexChanged(object sender, EventArgs e)
{

getData(newsubgrpddl.SelectedItem.Text);


}

private void getData(string grades)
{
DataTable dt = new DataTable();
string connectionString = ConfigurationManager.AppSettings["ApplicationServices"];
SqlConnection myconnection = new SqlConnection(connectionString);
myconnection.Open();
SqlCommand sqlCmd = new SqlCommand("SELECT * from Admin_SubjectGroups WHERE SubjectGroupName = @SubjectGroupName", myconnection);
SqlDataAdapter sqlDa = new SqlDataAdapter(sqlCmd);
sqlCmd.Parameters.AddWithValue("@SubjectGroupName", subgrptxtbox.Text);
sqlDa.Fill(dt);
if (dt.Rows.Count > 0)
{
subprefixtxtbox.Text = dt.Rows[0]["subprefixtxtbox"].ToString();
}
myconnection.Close();
}

my tables:

Admin_SubjectGroups:

AuthorityCode int not null
SubjectGroupid int not null
SubjectGroupName nvarchar(50)


Admin_Subjects

SubjectGroupid not null
subjectprefix nvarchar(50)


Please can you help.. I have just tried the above code but its not displaying anything in the text boxwhen i select a value from dropdown list. Thanks