I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
Let's hope you have a written agreement. If you do and you have completed it, then you just write a letter showing the proof and make them pay. If they do not abide, then contact a local attorney and have them write a letter on your behalf. Usually that sort of clout helps.
If you do not have a written agreement, you will probably be paid the amount of your work -- i.e. your expenses and your hourly charge, etc.