Natoons does the file exist at the path indicated ? (helper.php)
If the file exists, it might have the wrong file permission. Make sure your webserver user is able to read the file. You might have executed Composer with the "wrong" command line user.
If the file doesn't exist, Composer might have crashed during the update. You can try running composer install to re-install the dependencies.
You could also try to run composer dump-autoload -a to re-create Composer boot files that indicate where to find that file.
