HELP! Out of Network Surgery?

In 2014 my wife got approval for a surgery from the insurance company, the surgeon, hospital, etc were all IN-NETWORK, all the appointments leading up to the surgery were IN-NETWORK. The Surgery was scheduled MONTHS in advance, and was performed in the beginning of January.

Recently the surgeon's office called and wanted to discuss our out of network payments, obviously we were shocked. She said they they recently decided to become out of network, and never informed us of any changes.

Now we have a bill for $33,000+ and we feel we shouldn't have to pay anything, because the surgery was approved by our insurance for $0 when we were in network- and the surgeon just decided to become out of network after we'd done all the paperwork.

Do I have a leg to stand on legally here? I'm also in NY.