Kesalahan eror pada $_session

Undefined variable: _SESSION in C:\xampp\htdocs\invent\Admin\pages\masuk_data_bara

<?php
                                    include "../../config/connect.php";
									$id_barang = $_SESSION['id_barang'];
                                    $query = mysql_query("select * from stok  WHERE id_barang='$id_barang'");
                                    $row = mysql_fetch_array($query);
                                    ?>

ng.php on line 18 Id Barang Baru