search and study "sassion variables" and site master (optional)
when logging in, use as session variable for storing login info,
every page or your master page must check for the that variable in session, if it is okay, it lets the user to load page, otherwise it redirects it to login page
in login page:
Session.add("myvarableName", "Value")
in master page, or in every page load
c#
if (Session["myvariableName"]!=Null & Session["myvariable"]!= "value") Response.Redirect("Login.aspx",true);
vb .net
if Session("myvariableName") isnot nothing and Session("myvariable") = "value"
Response.Redirect("Login.aspx",true)
end if