Change cover without informing a client!

We have Uncapped data for 12 years and yesterday my contract was changed without my authorization and approval to pre-paid! I was not even informed of the change or received notice - this line is used for my work purposes, which I use to work from home. How can you just change cover without informing a person - I need a manager to make contact with me and have this sorted out a.s.a.p - Claudette Greeff ***