I have been an FNB client for decades, never skipping on payments

I have been an FNB client for decades, never skipping on payments. I recently finished at my employment and requested a payment break, as they payout will take a few months at most, I sent in relevant documents. This morning I receive a message saying I must immediately pay a certain amount.I contacted the bank again,get transferred from 1 to another and then threatened with legal action. If I didnt contact them its 1 thing,but I contacted them. What do I do,as a loyal client, for decades.