Hello MadameLaBouche
Please kindly call the customer service number that is stated on the invoice. If the invoice was sent from SBB, use: +41 848 00 11 33.
The number that @RedBaron mentioned can help you with general questions regarding train travel; however, they cannot help you with questions concerning invoices for travel without a valid ticket. These are handled by a dedicated team, the Servicecenter Einnahmen; for data protection reasons, only they have access to your customer file.
Kind regards,
Roland