Ciao,
prova a cambiare il file form in questo modo....

Codice PHP:
<?php

include 'config.php';

if(isset($_POST[btn_login]))
{
    $username=$_POST['username'];
    $password=$_POST['password'];

    if ($username==$usern && $password==$passd)
    {
        break();
    } 
    else
    {
        echo"ACCESSO FALLITO";
    }


}

?>

<html>
<head>
<title>Protezione pagina</title>
</head>
<body>


Protezione pagina "pag_protetta"</p>

<form method="post" action="form.php">



Username: <input type="text" name="username" size="20"></p>


Password: <input type="password" name="password" size="20"></p>


<input type="submit" name="btn_login" value="Login">
<input type="reset" value="Annulla">
</form>
</body>
</html>