Web Forms :: Display JavaScript Alert MessageBox On Validation Fail

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

refer [URL] .... 

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();

[Code] .......