Thanks, you need to go through a procedure in that case - which starts with a formal complaint.
If you make a formal complaint (you don't say which provider you are with but their complaints email address will be available online if you do a search) and if that does not resolve the matter ask them for a "deadlock letter", which is a letter giving their final response.
Once you have the deadlock letter the next step would be to escalate this to Ofgem (their regulator) - they will investigate and liaise with the energy company which would hopefully result in a refund of any over payments you have made and a reduction in the bill (or a cancellation). They can order the energy company to make a financial award for inconvenience if they have acted badly.
The service is covered by the Consumer Rights Act 2015 so you have a right to expect a reasonable service which is carried out with care and skill - if you do not then the regulator will step in but it also means you can sue for any damages or losses incurred as a result of the energy company's conduct.
You can make the complaint to the Ombudsman here :
https://www.ombudsman-services.org/sectors/energy or you can send them an email to : [email protected]energy.org
If they sue then the court will send you a claim form and a response pack - you must fill in the response pack to confirm you want to defend the claim and then return it to the court. If you ignored it or delayed then the claimant can request default judgment (which means a county court judgment, or a CCJ).
I hope this helps - if you can please accept the answer and give me a 5 star rating (there should be a button at the top of your screen to do this), I can answer follow up Q&A's at no extra charge and Just Answer will credit me for helping you today.