Employer did not pay my health premiums the month i gave birth...

So my health premiums are deducted from my salary and the month i gave birth due to an "administrative error" my employer didnt pay my premiums and my coverage lapsed (i work for a very large cooperation and someone obviously dropped the ball)...they wrote me a letter telling me that the error has been rectified and that my coverage has been reinstated...but they have just informed me two month later that the insurance carrier has denied the reinstatement...the month that my coverage lapsed is the month i had an emergency c section so if carrier doesnt pay then i will be on hook for over 50k in hospital bills. I still work for this employer and still very much want my job but i am at a complete loss at what next step to take.

UPDATE - i called HR and they say that the carrier is stating that too much time has passed for them to reinstate me...the delay was on the part of my company's HR. I went back and forth with HR for about 2months before they finally realized that the error was on their end. So because of that delay they are saying the carrier is refusing to reinstate me. I have everything from my HR in writing, that they made a mistake and will get me reinstated.

Im in massachusettes.