session lost when paypal return url .net

Question

code of return url like:

 

if (Convert.ToString(Session["login_nm"]) != null && Convert.ToString(Session["login_nm"]) != "")

{

lbl_name.Text = (Session["login_nm"]).ToString();

lbl_msg.Text = "Thank You for shoping with us.";

}

 

 

when pay pal return  thank u page 

 

then Session["login_nm"] value null

 

how to get it