Can a person be denied medical treatment?

My friend has about $5,000.00 in unpaid medical bills because she has no insurance and needed gall stones removed. She fell down the stairs and hurt her back and went to the walk in clinic to make sure she didn't really hurt herself and she said that they said they couldn't look at her because of the past medical bills. I thought it was illegal to deny anyone medical treatment regardless of the severity. I don't know if the state makes a difference or not with the laws but I live in Connecticut.