Account in rear

Good day. I've sign up with rain internet service to deduct every month money from my account. All the months they deduct money and now all of a sudden they stop the debit order. Now they want to sue me for money that my account is in rear. I didn't stop the debit order they did it. They service is very bad. When you send them a email to phone you back they never phone back. How must we sort the problem out if they can't to there job properly.

Please help. Thank you