Pre-auth was approved but they denied the claim after surgery - is this even legal?

I'm completely lost here and honestly kind of panicking. Back in November I had a knee surgery scheduled. Called my insurance, went through the whole pre-authorization process, got a confirmation number, the whole thing. They told me the procedure was approved. I even called twice to double-check because I was nervous about it.

Now it's been 6 weeks and I just got an EOB saying the claim was denied. The reason they gave is "procedure not medically necessary." That's the exact opposite of what they told me when they approved the pre-auth. I owe over $14,000 right now if this doesn't get resolved.

Has anyone dealt with this before? Is this something I can actually fight? My surgeon's office said they've seen this happen and that I should file an appeal but I honestly don't know where to start. Do I need a lawyer or can i do this myself?