Invoice not sent on time

I sent two emails requesting an invoice as they had not sent it and I could not view it from their website. They did not reply to or send it, so I made a payment based on my last month's invoice. A week after, I was sent a notice that I was short and they are going to cut off my electricity. I have sent they emails to show that I have been asking for an invoice before I made payment but they still insist that I should make payment, this is their mistake but they want me to suffer for it.