Data Use and Excessively high Bill

Xfinity says my cell data use went from my normal 3 to 30 GBs and billed me for $740 instead of my normal $40. I've been with them for years and seldom have gone over my 3 gb allowance. I'm 88 years old and seldom leave my house so always am connected to my home wifi. Based on repeated calls thet say I used the data. I gotta pay.