ER visit denied - they say it wasnt a real emergency

Went to the ER with severe chest pain. Turns out it was a panic attack, not a heart attack. Now my insurance denied the claim saying the visit "did not meet the criteria for emergency care."

How was I supposed to know it wasnt my heart? I was terrified. The bill is $6,200. How do I appeal this? Its ridiculous that they can deny based on the diagnosis rather than the symptoms.