collection agency closed, letter returned

I was treated in the emergency room without insurance but received HCAP, basically charity forgiveness. when youre broke but not broke enough for medicaid. It took the hospital over 90 days to approve the "coverage" so the attending physician services billed me. I made them aware I was applying for the HCAP and waiting for approval. the day i received my HCAP approval I got my first collection agency letter. of course they refused to even listen to what I was saying and the physicians group wouldnt even review the HCAP letter despite the hospital showing a zero balance. I understand individual drs can bill separately, they just never even attempted to receive payment under the HCAP before collections. I sent  a letter to the agency listed as having the account for debt validation and the letter was returned as attempted-not known- unable to forward. Yelp shows the business as closed and this is the only agency showing ownership of the account on my CR. Any suggestions on what to do next? Not familiar with medical debt, any help is greatly appreciated