I feel for you OP. Must have been quite a shock to think you may owe that much.
But help is available. Please consider contacting Citizens' Advice. They have whole specialist energy sections who, by telephone, help people dealing with ALL of the queries and worries you have about your energy supply, boiler etc. They are highly trained and provide a lot of assistance - including dealing with the energy company on your behalf. My friend works for them so I know what they provide - and it's an excellent, high quality service. There's also a lot of info on energy on their website.
I have copied the following details from their website. Very best of luck in getting this sorted soon.
Citizens Advice consumer helpline: 0808 223 1133
Relay UK - if you can't hear or speak on the phone, you can type what you want to say: 18001 then 0808 223 1133
You can use Relay UK with an app or a textphone. There’s no extra charge to use it. Find out how to use Relay UK on the Relay UK website.
Lines are open Monday to Friday, 9am to 5pm. Lines are closed on bank holidays.
To contact a Welsh-speaking adviser: 0808 223 1144
An adviser will answer your call as soon as possible, usually within a few minutes. Once you're speaking to an adviser your call should take an average of 8 to 10 minutes.
Calls from mobiles and landlines are free.
Chat with us online
You can also talk about your consumer issue with a trained adviser online. We’ll try to help you solve your problem or make good progress towards it.
Chat is usually available 9am to 5pm, Monday to Friday.