The health care law offers rights and protections that make coverage more fair and easy to understand. Some rights and protections apply to plans in the Health Insurance Marketplace or other individual insurance, some apply to job-based plans, and some apply to all health coverage. The protections outlined below may not apply to grandfathered health insurance plans.
From FindLaw.com:

A health insurance claim denial can threaten your financial security and ability to access medical care, but it doesn't need to be the end of the story. The following article reviews the reasons why a health insurance claim may be denied and actions you should take following a denial.

Reasons for Health Insurance Claim Denial
The first step will be to identify the insurer's reason for denying your claim. The insurer, your doctor, or the hospital may be able to help explain the insurer's stated reasons for refusing coverage. The reasoning behind a denial will provide some insight as to your options. The most common reasons for claim denials include:
• Denial of services or procedures not covered by your policy;
• Denial because a procedure is considered experimental, cosmetic, investigational, or not medically necessary;
• Denial because a referral or pre-authorization was required;
• Denial because you used an out-of-network provider;
• Denial for typographical errors;
• Denial for timeliness; or
• Denial due to policy limitations.

Many of these reasons for denial can be addressed, either by appealing the insurer's decision, or by contacting the doctor or hospital to make corrections to the documents submitted.