how to stay on modal pop up page if any wrong validation occurs?

Hi all,

Hope doing well,

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 desing code:

<div>
   <asp:UpdatePanel ID="UpdatePanel1" runat="server">
   <Triggers>
       <asp:PostBackTrigger ControlID="btnExcel"/>
    </Triggers>
    <ContentTemplate>
    <asp:Panel ID="Panel3" runat="server" Width="300px">
            <div style="text-align:center">
                <table style="width:100%; background-color:White;">
                    <tr>
                        <td colspan="2" align="center">
                             <asp:Image ID="Image1" runat="server" ImageUrl="~/images/login2_new.jpg" Width="40px"  />
    
     
                    <asp:Image ID="Image5" runat="server" ImageUrl="~/images/Picture5_new.png" Width="160px" Height="30px" vspace="8"/>
                        </td>
                        <td valign="top">
                            <asp:ImageButton ID="BtnClose" runat="server" ImageUrl="~/images/CrossImage.jpg" />
                        </td>
                    </tr>
                    <tr>
                        <td colspan="2" align="center">
                            <asp:Label ID="Label3" runat="server" Text="Captcha Login" CssClass="printformat"></asp:Label>
                        </td>
                    </tr>
                    <tr>
                        <td>
                            <asp:Label ID="Label4" runat="server" Text=""></asp:Label>
                        </td>
                    </tr>
                    <tr>
                        <td>
                            <asp:Label ID="Label5" runat="server" Text=""></asp:Label>
                        </td>
                    </tr>
                    <tr>
                        <td align="center" colspan="2">
                          <div id="showcaptcha" runat="server">
                        <asp:CaptchaControl ID="Captcha1" runat="server"

                         CaptchaBackgroundNoise="Low" CaptchaLength="5"

                         CaptchaHeight="60" CaptchaWidth="150"

                         CaptchaLineNoise="None" CaptchaMinTimeout="5"

                         CaptchaMaxTimeout="240" FontColor = "#529E00" /><br />
                            <asp:TextBox ID="Txtcaptcha" runat="server" Width="200px" CssClass="textbox150"></asp:TextBox>
                       </div>
                        </td>
                        <td>
                           
                        </td>
                    </tr>
                    <tr>
                        <td align="center" colspan="2">
                            <fieldset>
                                <asp:Button ID="btnExcel" runat="server" Text="verify" CssClass="button_57"
                                   onclick="btnExcel_Click" />
                            </fieldset>
                        </td>
                        <td>
                        </td>
                    </tr>
                    <tr>
                        <td>
                            <asp:Label ID="Label6" runat="server" Text=""></asp:Label>
                        </td>
                    </tr>
                    <tr>
                        <td>
                            <asp:Label ID="Label9" runat="server" Text=""></asp:Label>
                            <asp:Label ID="Label10" runat="server" Text=""></asp:Label>
                        </td>
                    </tr>
                </table>
                
                </div >
                 </asp:Panel>
                 </ContentTemplate>
                 </asp:UpdatePanel>
                 <ajt:ModalPopupExtender ID="ModalPopupExtender1" BackgroundCssClass="modalpopup" PopupControlID="panel3"  CancelControlID="BtnClose"  TargetControlID="Label7" runat="server">
   </ajt:ModalPopupExtender>

</div>

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.

Thanks