Redirection after loggin in not working

private void imgBtnLogin_Click(object sender, ImageClickEventArgs e)
   {
      FormsAuthentication.Initialize();
       SqlConnection con = new SqlConnection("data source=.; initial catalog = AxaStock; integrated security = true");
       SqlCommand cmd = con.CreateCommand();
       cmd.CommandText = "select r.nomRole from Collaborateur c, Role r where r.idRole=c.idRole and matricule=@matricule and password=@password";
       cmd.Parameters.Add("@matricule", SqlDbType.VarChar, 64).Value = txtLogin.Text;
       cmd.Parameters.Add("@password", SqlDbType.VarChar, 128).Value = txtPassword.Text;

       con.Open();
       SqlDataReader reader = cmd.ExecuteReader();
       if (reader.Read())
       {
           FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(1, txtLogin.Text,
               DateTime.Now,
               DateTime.Now.AddMinutes(30),
               true,
               reader.GetString(0),
               FormsAuthentication.FormsCookiePath);
           string hash = FormsAuthentication.Encrypt(ticket);
           HttpCookie cookie = new HttpCookie(FormsAuthentication.FormsCookieName, hash);
           if (ticket.IsPersistent) cookie.Expires = ticket.Expiration;
           Response.Cookies.Add(cookie);
           string returnUrl = Request.QueryString["ReturnUrl"];


           if (returnUrl == null)
           {
               returnUrl = "/";

           }
       }
       else
       {
           lblError.Text = "Matricule / mot de passe incorrect. Réssayez !";
           lblError.Visible = true;
       }
       reader.Close();
       con.Close();


   }