Facebook登录-无效的访问令牌

突然，我的网页上的facebook登录停止工作-它返回消息invalid access token。几个月前，它运行得很好。你能帮帮我吗?我使用php如下:

$facebook = new Facebook(array(
  'appId'  => 'xxx',
  'secret' => 'xxx',
));

$userfb = $facebook->getUser();
if ($userfb) {
    try { 
        $user_profile = $facebook->api('/me');

    } catch (FacebookApiException $e) {
        $userfb = null;
    }
}

if ($userfb==null) {

    $args = array('scope' => 'email');
    echo "<tr>";
    echo '<td><a href="' . $facebook->getLoginUrl($args) . '"><img src="fblogin.png" border="0" width="60" height="20"></a></td>';

    echo "</tr>";

 }