Automatically redirect user when session timout

You can create a BasePae class which inherits System.Web.UI.Page. Override the OnInit method and write appropriate code in that which will redirect to default page.

public class BasePage : System.Web.UI.Page
{
    protected override void OnInit(EventArgs e)
    {
        if (Session["UserName"] == null)
        {
            Response.Redirect("~/LoginPage.aspx");
        }
    }
}

And inherit all your ASPX pages from BasePage class, instead of System.Web.UI.Page like this:-

public partial class HomePage : BasePage
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
}

And finally in your login page, simply set Session["UserName"] with user's name.