What can I do regarding a wrongful collection from malpractice?

Two years ago, I was seen in an ER where the attending physician belittled me and never examined me but said I was crying for nothing and probably had the stomach bug. I went to another ER and was diagnosed as having excessive acid buildup in my stomach requiring a prescription antacid. I immediately contacted the first hospital after receiving the diagnosis. They dismissed all charges from my visit there and informed me the doctor who saw me was fired from their facility. They were contacted (2 years ago), made aware of the situation and told me that I would not be billed and the charge dismissed. I have not heard nor received anything since until I received a letter from a collections agency.