I just had the same thing happen to me recently and here's what the insurance company told me.
Since I had insurance, as long as the doctor billed the insurance company within a reasonable amount of time, which they did, then I am responsible for my portion. If the doctor tried to bill the insurance company past a certain amount of time, neither they nor I would be responsible to pay. The insurance company did not get specific as to how much time was too much.
You will have to pay the bill. Likely, the doctor audited their records and found they never billed you. You can pay in installments. I just started paying $25/month three months in a row, and they just bill me $25/month now, until it is paid off. Most people are reasonable if you are also reasonable.