Still billed after cancelation

I really frustrated I have canceled my contract in March 24, and recieved cancel confirmation e-mail from X-finity on the same day, but it still charged in April and May. Why you have full charged after my cancel period was April 2nd, but it continue to charge and didn't close my account. I would request to adjustment and credit refund which I didn't use after my cancelation period. Please tell me how to solve this kind of issue?