They're bastards. They said I owed them £9000 because I didn't tell them about my new job. I did, I could tell them the exact date I made the call. They kept denying there was a call made, then said there was, but they wouldn't listen to it - id have to appeal, which would take up to a year, and wouldn't get paid until then.
I wanted to die I was so scared me and DD would become homeless. My MP tried to get it sorted as best as he could. What really helped (after advice from MN) was emailing the chief exec. About a week after emailing him, I got a letter of apology back, said I was right, wrote of the (imaginary) debt and they gave me £50 to say sorry (which was a bloody insult tbh).
So try them, ask for the department which deal with vulnerable people (I can't remember the name but they'll know if you say that - they should offer it to you automatically but they don't. They asked me very suspiciously about how I had found out about it - I actually knew because a HMRC worker messaged me on here about it). They will run through a payment plan for you which will help in the short term. That should get sorted within a few days. In the longer term, if you don't actually owe anything get onto your MP/ chief exec.
Now going through the same bloody thing with housing benefit. So will have to annoy my MP again, and get onto the head of the council. It does work, I promise. I swear they just try to take advantage of vulnerable people who don't know the processes (because they're not publicised). I've always been completely transparent and honest about the money I earn. I can't bloody wait until I earn enough to not depend on these awful government departments who treat you like you're an idiot, and a sponger.
to you OP. It's so stressful.