ASP.NET

<asp:ModalPopupExtender I
     ID="MPEProphetAndLoss1" 
     runat="server" 
     PopupControlID="divProphetAndLoss1"
     TargetControlID="LinkButtonIntroductionText"
     CancelControlID = "bt_Modal_Cancel"
>

<div id="divProphetAndLoss1" runat="server" class="PopUpTitle" // <- test your css class

protected void btnIntroductionText_Click(object sender, EventArgs e)
{
MPEProphetAndLoss1.Show(); // Show or Hide
}

Sys.WebForms.PageRequestManager.getInstance()._registerDisposeScript('parentupdatepanelid', 'var component = $find("componentid"); if (component != null)component.dispose()';

function Sys$WebForms$PageRequestManager$_updatePanel(updatePanelElement, rendering) {
    for (var updatePanelID in this._scriptDisposes) {
        if (this._elementContains(updatePanelElement, document.getElementById(updatePanelID))) {
            var disposeScripts = this._scriptDisposes[updatePanelID];
            for (var i = 0, l = disposeScripts.length; i < l; i++) {
                eval(disposeScripts[i]);
            }
            delete this._scriptDisposes[updatePanelID];
        }
    }
    this._destroyTree(updatePanelElement);
    updatePanelElement.innerHTML = rendering;
}

if (Session["UserName"] == null) Response.Redirect("~/loggingpage.aspx");