Health insurance denied my MRI claim - no real explanation. What do I do?

I had an MRI last month that my doctor ordered because of chronic back pain. My insurance approved the pre-authorization but now they're denying the claim saying it wasn't "medically necessary".

The denial letter is so vague. Just says it doesnt meet their criteria. My doctor is confused too since they approved it beforehand.

This is a $2,800 bill I definitely cannot afford to pay out of pocket. Has anyone dealt with this? Do I appeal? Get a lawyer? Im so lost and stressed about this.