First, if this was an emergency trip to the hospital, your Primary Care doctor must obtain the authorization number for you. They direct your care. I just got out of the hospital last week and saw my regular doctor right away, told her what had happened to me (my throat swelled shut) and she immediately got me an authorization number for the hospital stay, the emergency room, the emergency room doctor, the cardiologist who saw me and all my lab work within 24 hours of my visit to her. I received the number in the mail the next day.That number she got will now attach to every bill. So, first thing is to call your regular doctor. Let me know if this doesn't help or you were in the hospital for something other than an emergency. Don't want you to get stuck with a bill if I can help you. Keep the letter with the number on it. Send a copy to the hospital.
The hospital should also have called your insurance company as soon as you got there and reported to them why your stay was necessary.