Hospital bill went to debt collector?

I went to the hospital last year, and after receiving a letter I reconciled with them on an amount and payment plan. I get another letter this year saying that my bill has been forwared to a collection agency AR Resources. I call the hospital, turns out the amount they were dealing with was for their bill only, the doctor had a separate bill and that is the one that was sent to collections. I call the doctor's billing office and they tell me there's nothing they can do since it's already out of their system. I tried calling AR resources if I can settle something or if they can give it back to the doctors so I can try to work something out. They insist they cannot.

What can I do? It's $1,400. What happens if I don't pay them?