Insurance took back payment of major surgery?

hi everyone- any help is greatly appreciated.

I left my (salaried) career in June of last year. My insurance through that career lasted until September 2021. My new insurance from my new job began in September of 2021. I was in a major accident in June of 2021, and had a major surgery. My insurance paid for the surgery, I had a small ($1,000) balance with the surgery center that I then paid.

Now, September 2022, I was served with a summons. The hospital is now suing me for $12k. The summons was the FIRST I had heard of the debt, I had no idea it was happening. I called the firm that represents the hospital to set up a plan so I could move forward. They then told me that my insurance revoked payment months after they’d already paid for the surgery, so the debt was now entirely mine.

Obviously I have to call the insurance and figure out why they revoked payment. I don’t have $12k to drop right now, but will pay the debt if I need to- i just don’t feel I have to. The firm was super kind and told me what I needed to do (calling insurance etc) and let her attorney know we were working it out. I’m very appreciative of that, but also very nervous to deal with insurance.

Does anyone have advice on how to talk to the insurance company, the correct questions to ask, and any thoughts on what I might be looking at in the long haul?

Thank you!