Selling of flat by society without owners permission

My parents owned a room in a chawl in Thane. Society decided to convert chawl to building. My parents and other few opposed this decision. At that time my parents resided in vashi new Bombay. Due to emergency parents had to go to native place. During this period the builder and the society officials then broke into our room and removed our property from the house and started the construction in around 1987. When my parents returned they lodged a complaint against secretary and chairman. The building was constructed and now my parents asked for possession.They now asked for illegitimate maintenance tax which was a big sum in 2004. They were asked to produce records for this maintenance, which they were not able to. Due to financial problems my father was not able to fight legally as previous lawyers were bribed by the builder. Society officials then gave room on rent for years.Now they have sold the room to a third party without intimating us. Please suggest me what can I do in such situation to get my room back Very much thanks in advance.