Billed after contract ended

I've been using using the contratct for 50GB anytime data and now that the contratct is over they changed it to 50GB day+50GB night data and is costing almost double than the original contract without even phoning me or giving me notice that it changed. I tried to cancel the contract and have not been using it for several months now and yet they still haven't cancelled it and kept billing me, so I refuse to pay the amount that I've been billed for while i haven't been using it at all and still they haven't contacted me or stopped the contract. All I want is to have it cancelled and not pay for what I haven't been using over the past few months since I've been unemployed for almost 3 years now and can NOT afford to pay them at all.