Call your insurance company. It's possible that the hospital didn't properly file paperwork with your insurance company. Your insurer can check to see if a claim was filed and advise you how to proceed. You may have to call the hospital billing dept to proceed on this.
Also check your policy. If your haven't met your deductible or the insurer only covers a certain percentage of ER services, for instance, then you may have a balance to pay off. Ah, the fun -- going through insurance policy papers!
If your insurance is through your employer, you may contact HR if things stall out. Many employers have HR Benefits staff who can advise you on how to deal with the insurer.