Show alert error message when certain condition arises

I have the following code on page load that I use to show an error alert:

 protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                if (Request.QueryString["message"] == "noemployees")
                    AlertDanger.Visible = true;

---

This is where the error is called. The page reloads and the error is shown.

if (payFitments == null)
{
     Response.Redirect("Default?message=noemployees");
}

I have the following markup

 <script type="text/javascript">       
        $(document).ready(function () {
            window.setTimeout(function () {
                $(".alert").fadeTo(1500, 0).slideUp(500, function () {
                    $(this).remove();
                });
            }, 3000);
        });
       -------------------------
<div runat="server" visible="false" id="AlertDanger" class="alert alert-danger">
        <a href="#" class="close" data-dismiss="alert">&times;</a>
        <strong>You must have at least one employee to process</strong>
    </div>

How do I show this message without having to load the default page again? Not seeing examples on the web that show this clearly.

from Show alert error message when certain condition arises