Poor service

I took the 3gb service for my mtn Sim on prepaid MOnth to month every month it was working and fine until I told them not to top up this MO th as I have enough data rolled over for me to last next month or 2 so they canceled the service and I lost my data roll over, this is rediculous seriously they want me to pay for the next month then I only will be available to axxess my roll over data