MTN poor service

I've been a customer for more than 25 years and very happy. I haven't upgraded on my existing contract and you've notified me that I am on month-to-month. Then yesterday I receive an SMS saying you have not received payment and today I cannot make calls. I have a debit order with MTN. Why do I need to make any payments? I never had to do that in the past when I was moved to month-to-month. I am really disappointed. I did not upgrade yet because I haven't had the time. You might just have made up my.mind for me in terms of what I am going to do. I'll move to Vodacom. Thank you.