AEP Energy review: 15 year old $900 bill and being threatened disconnection

I have been an AEP customer for 16+ years. After closing on my new home and transferring service to my new home, I received a bill for nearly $900. After calling AEP, I was told that this was a bill from 15+ years ago when I was a college student! I didn’t even reside in the apartment I’m being billed for the last 6 months of its lease (it was vacant mid December 2009-June 2010). I resided there around August 2009-mid December 2009.
I have had an account with AEP all of these years and no one can tell me why I am just now receiving this bill nor can they give me an itemized/usage receipt/report. I have made countless calls over the last 3 weeks to dispute this and have been told by multiple supervisors that they’ll have their supervisor call me and then I don’t receive a call back and have to start all over again. I am a single mother of 5 who pays my bills and have been paying AEP all these years with no knowledge of this ancient bill.
Additionally and most importantly, I have an 8 year old daughter who has a life threatening disease, of which I’ve also explained to them, but because she doesn’t require an oxygen tank, this is of no importance to them.
They are now telling me that my service will be disconnected TOMORROW if the bill isn’t settled. I do not have this kind of money set aside for this. AEP is a monopoly in my area and I have no other option for power. This is absolutely despicable.