You should not delay pursuing this. Depending on the insurance contract you may (probably are) bound to binding arbitration and other limiting clauses.
My suggestion is to immediately file a complaint with the California Insurance commissioner's office. They are very helpful at investigating the claim and giving you their view. Usually it is informative but not too helpful in actually getting the insurance company to pay the benefits.
You should also immediately consult with a health care law attorney so that he can review your health insurance policy and your facts and give you advice as to time limits and so forth.
The foregoing should not be considered legal advice as you need to consult with an attorney with full knowledge of the facts and your insurance policy/contract.
You probably can't use, and may have to arbitrate, and you can't do anything until you've exhausted the insurance company's grievance/appeal process, which they should inform you about on their denial of benefits.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.