Just got insurance denied on a medical procedure scheduled for tomorrow

Everything online says that the procedure should be covered, and that the hospital accepts my insurance. I was also referred by my primary care doctor to this specific surgeon, after she asked what insurance plan I had, which made me assume it’d be covered.

The insurance denial says the doctor is not in network, and there are other doctors that are in network. As a result, it says, the procedure is not medically necessary. (These, to me, feel like two different things?)

Anyway. I’m trying not to freak out. I will appeal it. But I know almost nothing about insurance, or about processes like these. What are my odds of being ok, and being absolutely screwed?

The plan is to go through with the surgery tomorrow, because it’s been in the works for nearly two years now. But I’m stressing and would appreciate any insight anyone has to offer?