My insurance covered my orthodontics as a teen.
They covered all routine dental work.
They covered a procedure in which I had a baby tooth pulled, because there was no adult tooth underneath.
They paid for the retainer that I had to wear in order to prepare my mouth for the eventual 'fake tooth' that would be installed in the 'hole'.

They now say that they will not pay for the procedure in which the fake tooth will actually *be* installed, nor for any related lab tests and referrals, calling it a 'cosmetic procedure'.

What the h*ll!!
If this is cosmetic, then what, exactly is orthodontics defined as?
And how about the pulling of the tooth in the first place?

Why will they not pay for the final result?

(any answer appreciated, since I now have a big hole in my mouth, and can't afford the several-thousand dollars to fix this!)

Read your insurance policy. The company will pay for procedures that are covered, but most policies do not pay for cosmetic work. Sometimes it is a matter of interpretation whether some procedure is cosmetic or medically required.

Your orthodontist should be able to interpret your policy. If the amount of money involved is not large, it is probably best for you to pay it. But if it is a significant amount and you don't agree with the interpretations, you may want to have a lawyer look at the policy and try to arrange for payment by the insurance company.