Defamation case on developer for balkmailing and stooping the services

I had given development of app for my company to other registered company and they delayed the time line by two months however I given time to make but when it's launched for trail and users starred to join and play then suddenly they stopped the server and start blackmailing me. By this act my company's name is on stake and I wish to send them a legal notice for person and company defamation. I have all the communication are in place in written since beginning