C# - Database To DropDownList And AutoPostBack To Label

I have a code like this

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
string strConnectionString = ConfigurationManager.ConnectionStrings["SqlServerCstr"].ConnectionString;
SqlConnection myConnection = new SqlConnection(strConnectionString);
myConnection.Open();
string musisim = DropDownList1.SelectedItem.Value;
SqlCommand cmd = new SqlCommand("select B.HESAP_NO FROM YAZ..MARDATA.S_TEKLIF B WHERE B.MUS_K_ISIM = DropDownList1.SelectedItem.Value", myConnection);
Label1.Text = cmd.ExecuteReader().ToString();
myConnection.Close();

i have a customer name as "MUS_K_ISIM" and his number as "HESAP_NO" All i want is, (autopostback is true) automaticly getting label "HESAP_NO" with who has this number "MUS_K_ISIM" in Dropdownlist. How can i do that? Error: Description: An unhandled exception occurred during the execution of the current web request. review the stack trace for more information about the error and where it originated in the code.