Get some solid proof showing that you are the original author of the content. Write a letter to them explaining that they are infringing on your intellectual property and that you demand that they cease immediately. Im not sure where you are from but a lot of countries have rules stipulating certain rules whether or not they realize that they are infringing on your rights.
An idea is to ask them what their opinion is on the matter. without mentioning compensation at first. Just to see if they are willing to admit the infringement.
Depending on their answer (or absence of such) you can either demand compensation yourself or let a lawyer take a look at the case.