using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public partial class Reusable_Messagebox_using_Ajax_Modelpopupextender : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        if (txtUname.Text.Trim() == string.Empty)
        {
            PopupMsg.Message = "Enter Username or Email";
            PopupMsg.ShowPopUp();
            return;
        }
    }
}

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="wucPopupMessageBox.ascx.cs" Inherits="wucPopupMessageBox" %>
<link href="css/ModelPopup.css" rel="stylesheet" type="text/css" />
<asp:LinkButton ID="LinkFake1" runat="server"></asp:LinkButton>
<asp:Panel ID="PanelMsg" runat="server" CssClass="modalPopupMsg" Style="display: none;">
<div class="popupHeader" style="text-align:left">
<span>
Message <span style="float: right">
<asp:ImageButton ID="imgPopupClose" runat="server" ImageUrl="~/images/close.png" /></span></span>
</div>
<div class="popupBody" style="text-align:left">
<table width="100%">       
<tr>
<td style="width: 60px" valign="middle" align="center">
<asp:Image ID="imgInfo" runat="server" ImageUrl="~/Images/Info-48x48.png" />
</td>
<td valign="middle" align="left">
<asp:Label ID="lblMsg" runat="server" Text=""></asp:Label>
</td>
</tr>
<tr>
<td colspan="2" align="right">
<asp:Button ID="btnOk" runat="server" Text="Ok" OnClick="btnOk_Click" />
</td>
</tr>
</table>
      
</div>
</asp:Panel>
<ajaxToolkit:ModalPopupExtender ID="PopupMsg" runat="server" DropShadow="false" PopupControlID="PanelMsg" 
CancelControlID="imgPopupClose" TargetControlID="LinkFake1" BackgroundCssClass="modalBackground">
</ajaxToolkit:ModalPopupExtender>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public partial class wucPopupMessageBox : System.Web.UI.UserControl
{
    public string Message { get; set; }
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    public void ShowPopUp()
    {
        lblMsg.Text = Message;
        PopupMsg.Show();
    }
    public void btnOk_Click(object sender, EventArgs e)
    {
        PopupMsg.Hide();
    }
}