you could add the following code at the top of the contact page:
<?php
if($_SERVER['REQUEST_METHOD'] == "POST") {
foreach( $_POST as $key=>$val) {
$_SESSION[$key] = $val;
}
}
?>
this will store the form values in session variables, then on the thank you page, you can use the session variables to output the information submitted by the form:
<?php echo((isset($_SESSION["Contact_Name"]))?$_SESSION["Contact_Name"]:"") ?>