Folder name should redirect to a page which is out side the folder

Use the session variable in the page_load:

protected void Page_Load(object sender, EventArgs e)
{
    Session["loggedin"] = "false";
}
... and check the login event. If user has pressed the login button then set the session variable "true".

protected void btnOK_Click(object sender, EventArgs e)
{
    Session["loggedin"] = "true";
    ......
   ......
}
When the page is redirected to another page then check the status of the session variable in page load event.

    try
    {

        if (Session["loggedin"].ToString() == "false")
        {
            Response.Write("<script> alert('Session Expires! please login first'); </script>");

            Response.Redirect("login.aspx");
            return;
        }

    }
    catch (Exception er)
    {
        Response.Redirect("login.aspx");
    }