How can we sue a debt settlement company?

Hello, recently we tried to do a credit card settlement using services of a debt settlement company. They were supposed to negotiate with the creditor, but after several months we figured out that they just collected our money (several thousand dollars) and did not negotiate anything. We stopped paying them, but now they wouldn't return the money we paid despite the fact that they provided no service. The contract we signed with them mentions initiation fee and monthly fee, but they retained much more; they also violated the contract. What can we do to get the money back? Thank you for your help.