Step one: retain an experienced lawyer.
If an insurance company has denied your claim, it's likely time to hire a lawyer, because litigation is going to be necessary. Now, there are two types of denied insurance claims that come up most often. A first party claim, which is where your own insurance company denies the claim that you've made against them. Something like a homeowner's claim. Here your attorney would file a breach of contract lawsuit against your own insurance company. The other situation is what's known as a third party claim. That's where another individual has committed an act against you that has caused you damage. In that instance, your attorney would sue that individual, and their insurance company would provide them with the defense, and ultimately pay any amounts they are deemed to owe.