Try to load the dropdown if it is not postback
if(!IsPostBack)
{
cmd = new SqlCommand("select RoleName from Roles where Active=1", con);
con.Open();
da = new SqlDataAdapter (cmd);
da.Fill (ds);
DropDownList1.DataSource = ds;
DropDownList1.DataTextField = ds.Tables[0].Columns[0].ToString ();
DropDownList1.DataBind();
con.Close();
}