Worst change of service provider

I recently changed my fibre service provider and assisted well but coming to the invoice which is no help at all i ran a small business and i budget my debit orders, so i got an invoice of R850 and i understand that there is pro-rata amount but if i see my invoice stating that amount to be debited is R850 then i make that my budget, so come month end the amount they tried to debit is R1700. Now my wifi is off cause of this invoice issue what do i do cause my business is on a stand still cause i cant load my programs.