A claim may be denied for several reasons. Many times it's a simple error that you (if you submitted the claim) or your provider made when submitting the claim. See Claim Filing Tips for a list of common mistakes. If your claim was denied because of incorrect filing, correct the error and resubmit the claim to your claims processor.
If you believe your claim was inappropriately denied, in whole or in part, you (or another appropriate party) may file an appeal.