Billing Question/Responsible party

[donaldkeithjohnson53@yaho]

I had a treatment plan that was gone over and approved. The plan as for $4,000 which was paid up front before the procedure. I recently received a bill for an additional $2,000, it summarized that insurance did not cover it. What is the proper thing moving forward? Resubmit to the insurance by my Dentist or myself?