Are you talking about the Premium Tax Credit that is available to taxpayers who acquire health insurance through the Marketplace under the Affordable Care Act (Obamacare)?
The Premium Tax Credit (PTC) is money sent from the government directly to your insurer to help pay for your health insurance premiums. That is, the government actually paid the money and has now determined (through form 8962) that you did not qualify for the Credit. Since they paid the money, they want it back - it doesn't matter whether or not you paid your part.
When you realized that you could no longer pay your part of the premium, did you contact the Marketplace and tell them? If you did, I would have thought that they would have stopped paying the PTC to your insurer, and this would have been reflected on your 1095-A. If this is the case, you need to contact the Marketplace and talk to someone about getting a corrected 1095-A.
But if you did not contact the Marketplace, then you never gave the Marketplace the chance to stop paying the PTC - so now you are on the hook for paying it back.
Sounds like you simply stopped paying your portion but did not cancel the insurance policy. After 3 months of non payment the policy lapsed automatically however you were still on the hook for the months part of the premium was being paid. When you signed up for the "obamacare" part of the instructions were to call if you had any kind of issues ... just ignoring the situation is never a good thing to do ... always stay ahead of the curve when you sign a contract.