Thank you for your reply.
But the Same error comes again. This is my Actual coding.
<?php session_start();
require_once('l2t_connect.php');
if($_GET['action'] == "send")
{
...

Please help me.

I got this error while uploading in Server. I developed this code in windows based. But the deployed server is Linux Server. In windows server i didn't get this session error.
...

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/leadtwot/public_html/staging/contact_us.php:1) in...