My dental work was pre-authorized and later denied by insurance. What recourse do I have?

I went to the dentist for a simple cleaning and was told that I needed a root scaling, which requires that I come back in two trips and costs $1000. The dentist obtained pre-authorization from my insurance company (Blue Cross/Blue Shield PPO--I have a copy of the pre-authorization paperwork), but after they did the work and submitted my claim, the insurance company rescinded the pre-authorization and said the work was not medically necessary. Now the dentist wants the $1000. The insurance company won't help me. What should I do?