mensaje: session_start() [function.session-start]: Cannot send session cache limiter

[systemar_marv]

a que se debe el mensaje subida al hosting:

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/sistemas/public_html/computerpower/mi-cuenta-cp.php:45) in /home/sistemas/public_html/computerpower/mi-cuenta-cp.php on line 48

ya que en localhost no me bota ningun error

<?php
// *** Validate request to login to this site.
if (!isset($_SESSION)) {
session_start();
}

$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($_GET['accesscheck'])) {
$_SESSION['PrevUrl'] = $_GET['accesscheck'];...................................