Modal pop extender not working

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C#

Ajax

Hi,

I am trying to use Modal Pop extender but somehow it is not working. The CSS class is working fine when the button is fired which activates the modal pop up but the panel which is the content of modal pop up is not appearing. I have seen many articles on it and almost I am doing the same thing still I am not able to get it. Can u pls help me ?
Here is my code :

protected void btn_add_new_Click(object sender, EventArgs e)
    {

        // HttpContext.Current.Response.Redirect("update-client.aspx");

        AjaxControlToolkit.ModalPopupExtender modalPop = new AjaxControlToolkit.ModalPopupExtender();
        modalPop.ID = "popUp";
        modalPop.PopupControlID = "pn_add_new_workOrder";
        modalPop.TargetControlID = "btn_add_new";
        modalPop.DropShadow = false;
        modalPop.BackgroundCssClass = "modalBackground";
        modalPop.CancelControlID = "btnCancel_new_inv";       
        this.pn_add_new_workOrder.Controls.Add(modalPop);
        modalPop.Show();       
    }