What if an employer fails to make insurance payments?

At the very least, you have the right to get back the premiums that were taken out of your check.  It's illegal for an employer to take anything out of your check, without giving you a true and accurate explanation of what the deduction is for.  Your employer broke that law.

It's a longer shot, to force your employer to pay the dental bill.  But it may be possible, if there's no good excuse for what happened here, especially if you can show that you would not have gone ahead with this dental work if you had known you didn't have the insurance.  There's an old court maxim:  "Equity will command that to be done, which should have been done."