You insurance agent. Your employer or your employees Personnel Department may be willing to help you by calling or writing the company. If the employer is the actual buyer of the coverage, the company will pay more attention to such a contact. If you bought your coverage through an insurance agent, they will often help you. Remember they are motivated to please you.
Doctors can also be an important source of help.
They will usually write a letter for you explaining why the claim should be covered, or why the company's reason for denying the claim is incorrect. Your case will be weakened, or even rejected entirely, if you do not comply with stated deadlines. Insist the insurer do the same. If you are a member of an HMO, the only agency which may help you is the Department of Corporations. Insist on filing a written complaint.