I'd go to a lawyer. If you have a signed contract and can prove that they agreed to pay, you can win in court. But firstly, if you haven't already, send a cease

I'd go to a lawyer. If you have a signed contract and can prove that they agreed to pay, you can win in court. But firstly, if you haven't already, send a cease and desist letter to them, mentioning the contract and that you will take the matter to court if they do not cooperate. I've been in this kind of trouble before and a C&D usually does the trick (as companies are intimidated by lawsuits). Also, try sending a DMCA takedown notice to the host, notifying them of your situation. They might pull the website in question.