Access Token is empty and getting error suddenly

Question

Although the issue is common and might be similar but none of the solution actually worked. My code

$facebook = new Facebook\Facebook([
    'app_id' =>  $config['fb_api'],
    'app_secret' =>  $config['fb_secret'],
    'default_graph_version' => 'v2.4',
    'cookie' => false,
]);
$helper = $facebook->getRedirectLoginHelper();

try {
    if (isset($_SESSION['facebook_access_token'])) {
        $accessToken = $_SESSION['facebook_access_token'];
    } else {
        $accessToken = $helper->getAccessToken();
    }
} catch(Facebook\Exceptions\FacebookResponseException $e) {
// When Graph returns an error
    echo 'Graph returned an error: ' . $e->getMessage();

exit;
} catch(Facebook\Exceptions\FacebookSDKException $e) {
// When validation fails or other local issues
    echo 'Facebook SDK returned an error: ' . $e->getMessage();
    exit;
}

But my $accessToken variable is empty. I checked dumping the variable and found its empty. It was working properly previously. Dont know what triggered and now I am getting following error.

Uncaught exception 'Facebook\Exceptions\FacebookAuthenticationException' with message 'An active access token must be used to query information about the current user