AJAX :: How To Keep Modal Popup Open If Validation Error Occurs

I am using one simple captcha form in modal popup so when i am inserting the captcha text in textbox if it is not validating

then in the same page i used one label to show the error "not validated". but when i click on the button modal popup is getting hide.

Here is my design code:

<div>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<Triggers>
<asp:PostBackTrigger ControlID="btnExcel"/>
</Triggers>
<ContentTemplate>
<asp:Panel ID="Panel3" runat="server" Width="300px">

[Code] ...

And here is my code behind:

 protected void btnExcel_Click(object sender, EventArgs e) {
Captcha1.ValidateCaptcha(Txtcaptcha.Text.Trim());    
if (Captcha1.UserValidated) {   
HiddenField1.Value = "0";   
VerifyPassword(txtUsername.Text, txtPassword.Text);
} else {
Label6.ForeColor = System.Drawing.Color.Red;  
Label6.Text = "Not validate";        
} }

i want it if it's not validate in the same modal popup page it should show the message.