Determining If User Is Log In Or Out?

I am currently developing a site for my OJT.I have created a login form for my site now I got confused on what to do with this one. I need to identify if the user has been log-in or log-out As the user inputted the exact username and password it will go to this line of codes in which will identity if the user has bees signout or not.

HttpCookie cookie = new HttpCookie("isLog");
cookie.Value = "1";
DateTime dtNow = DateTime.Now;
TimeSpan tsMinute = new TimeSpan(1, 1, 0, 0);
cookie.Expires = dtNow + tsMinute;
Response.Cookies.Add(cookie);
Response.Redirect("manageService.aspx");

And I have this function to determine if the user has been log-in or log-out.

protected Boolean isLog()
{
//String strCookieName = Request.QueryString["cookie"].ToString();
//Grab the cookie
HttpCookie cookie = Request.Cookies["isLog"];
String tmp = cookie.Value.ToString();
return (tmp != null);
}

Now, my problem here is that even though I inputted the correct username and password after reloading the page it will still asked for the user name and password(sends me to the login page). I already set a value for the "isLog" cookie but still it return a wrong info.