class error

<?php

session_start();

$go = new k_ex(); //ERROR OCCURS HERE!
$errormsg = "Thak You!You are logged out.<br>\n";
$go->error($errormsg,"..");



if(!isset($_SESSION['uvname'])){

session_destroy();
unset($_SESSION['uvname']);
session_unregister('uvname');


}
?>


when i call the k_ex class i get an error saying
Fatal error: Cannot instantiate non-existent class: k_ex in e:\..........\out.php on line 5

line 5 is the " $go = new k_ex(); //ERROR OCCURS HERE!".
the thing is that i used the same thing from some other file and i get no error!

please someone help.

You're not including your class to the script