Display JavaScript alert MessageBox on Validation fail in ASP.Net

hi 

I have changepass.aspx page that in this page i have 3 textbox and 1 captcha

refer

http://www.aspforums.net/Threads/172306/Implement-Change-Password-Feature-in-ASPNet-with-example/

I want when users enter their data correctly in message box show ="your password update" and if they enter wrong data in messagebox show="please enter oldpassword correctly"

so i use below code

 protected void Page_Load(object sender, EventArgs e)
    {


        if (Session["Message"] != null)
        {

         
            this.ClientScript.RegisterStartupScript(GetType(), "Javascript", "<script>alert('Yur password update correctly.')</script>");

            Session["Message"] = null;
        }

        if (Session["MessageError"] != null)
        {

          
            this.ClientScript.RegisterStartupScript(GetType(), "Javascript", "<script>alert('please enter old password correctly .')</script>");

            Session["MessageError"] = null;
        }

but when I enter data correct or wrong it didn't show any thing in message box

 protected void Imgpass_Click(object sender, ImageClickEventArgs e)
    {


         string data=Server.UrlDecode(Request.QueryString["Behcode"]);
         SqlCommand _cmd=new SqlCommand("changepassword",_cn);
         _cmd.CommandType=CommandType.StoredProcedure;
        _cn.Open();

        _cmd.Parameters.AddWithValue("@behcode",data);
        _cmd.Parameters.AddWithValue("@Oldpass",Txtold.Text);
        _cmd.Parameters.AddWithValue("@Newpass",Txtnew1.Text);
        bool isValid = aspcap.Validate(Txtc.Text.Trim());
        if (isValid)
        {
            int Result = _cmd.ExecuteNonQuery();
            if (Result > 0)
            {
                Session["Message"] = true;
                Txtc.Text = "";
                Txtnew1.Text = "";
                Txtnew2.Text = "";
                Txtold.Text = "";
            }
            else
            {
                Session["MessageError"] = true;
               
            }

            lblMessage.Text = "";
        }

        else
        {
            lblMessage.Text = "please enter correct number";
            lblMessage.ForeColor = Color.Red;
        }
        
    

            _cn.Close();

     

    }

Best Regards