You definitely need an attorney to work through this now. The dentist and the insurance company have dealt with you in bad faith. The insurance company probably did hope that you'd just go away. You aren't going to be able to fight this on your own, unfortunately. It's hard to say, without specialized knowledge of the laws in your area, to know who is responsible–the dentist, the old insurance company, or the new insurance company. Find a malpractice attorney who will work on contingency–don't pay an attorney anything up front.

You should probably contact an attorney who specializes in these matters.