Code Behind..........
....use any button click.......
Page.ClientScript.RegisterClientScriptBlock(GetType(), "JScript1", "window.scroll(0,0);", true);
lblEmpSelect.Text = "Please select employee in the list.";
ModalPopupExtender1.Show();
Panel1.Visible = true;
Code Behind...........
<asp:UpdatePanel runat="server" ID="Up">
<ContentTemplate>
<asp:Button ID="btnLogClose" runat="server" Style="display: none" />
<cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnLogClose"
PopupControlID="Panel1" CancelControlID="ImageButton3" BackgroundCssClass="modalBackground"
DropShadow="false" />
<asp:Panel ID="Panel1" runat="server" Width="550px" Height="100px" Style="display: none"
CssClass="modalPopup" BackColor="#FFFFFF" BorderColor="Gray" BorderStyle="Solid"
BorderWidth="5px">
<table width="100%" cellpadding="1" cellspacing="1">
<tr>
<td style="padding-right: 5px; width: 100%" align="right">
<table width="50%">
<tr>
<td style="width: 95%" align="left">
</td>
<td style="width: 5%" align="right">
<asp:ImageButton ImageUrl="~/Images/close.png" ID="ImageButton3" runat="server" />
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td align="center">
<b>
<asp:Label ID="lblEmpSelect" runat="server" Text="" ForeColor="Red"></asp:Label></b>
</td>
</tr>
</table>
</asp:Panel>
</ContentTemplate>
</asp:UpdatePanel>