I wouldn't be in too much of a hurry. You had a contract with the company to supply you with electricity. They 'cancelled' the contract, but did they have any right to do so? If it's their mistake then they should sort it out and if you were on a particular tariff and you do owe money to someone else, then they should make up the difference.
Once upon a time you could only buy electricity from your regional electricity board, a man came round every three months to read the meter, and shortly after that a paper bill arrived in the post which you took to the showroom, post office or bank and paid, as like as not in cash. Progress means we read our own meters, don't get paper bills and are encouraged to give them direct debits from our bank accounts. All that is in the name of saving the suppliers' costs, but in my view that means they have corresponding duties to get it right.
The successor to the local electricity board is now the default supplier where there is no other supplier, but they use that position to charge on a very high tariff. If they try that trick I think you should be asking for compensation from the company.
I once found myself in a position where I moved into a newish property and it turned out that the meter had never been registered on the system, so that the previous occupant had had several years free electricity. Being reasonably honest, I got the meter registered, and took out a contract with my supplier of choice, only to have the default supplier jump in and send me a very high bill based on the default tariff. I pointed out to them that the bill was based on estimated readings and the meter had never been officially read, so they didn't have any evidence that any electricity at all had been used during the time I was in the property. Funnily enough, they stopped chasing me after that.