Account Balance Inquiry

what a nightmare, the company just raised a invoice on my account for july in May and now they telling me my account is in arrears and my car won't be covered, I have been sending emails back and forth and the accounts personnel are just so arrogant and don't understand that the ssue is with their system, please make recommendation where I can complain and have this sorted because I am having a headache because of this company and their *********** accounts department