You file a lawsuit against them. For this amount of money, given that they are located in a different state or states than you, you could file in either state court (since they specifically hired you to do work for them, you could likely file in your state, in your county: their deliberate or intentional contracting with a company in your location could very likely be a sufficient contact with your state and county as to convey jurisdiction) or in federal court (under "diversity" jurisdiction for disputes between residents of different states). Filing a lawsuit is how you get money from someone who should have paid you but has not. If your company is an LLC or corporation, you *must* have an attorney to represent you; and for this amount of money, you *should* have a lawyer anyway. Therefore, retain an attorney to help you, and the lawyer can guide in the best court to file in, draft the legal papers, and file them.