I signed up with this service provider in June, and they debited my account for payment

I signed up with this service provider in June, and they debited my account for payment. Now they say I owe them because they have disconnected my internet connection. This is bad because I use the internet for work and at home. It is also very frustrating that I contacted them several times and they would resolve the issue, but the connection would still be poor! I am dissatisfied with the service I have received thus far.