Insurance approval, but didn't pay after surgery

[Alax2869]

Need some help from anyone that has experienced this.
I had a implant done over 2 years ago.
Doctors office submitted procedure to insurance for approval.
They got back approval letter so I was good to go.
Now, 2 years later I have a collection company calls me and said they have an account turned over to them for non payment of $47,000.
Insurance did not pay but 500.00 towards the procedure.
Waiting on call back now from Dr office to let them know what's going on.
I would have never had the surgery if I knew insurance wasn't going to pay, but how can they send approval letter, then not pay.?