What do I do when my health insurance won't pay for an out of network procedure I've already taken?

I just had a colonoscopy with a provider that is out of my network. I did not know that this was the case, they did not mention to me that they were and I did not pre-authorize with my health care provider. Now my health insurance says they will pay $600 and says I owe the $6000 remaining. Help!! What can I do?