You need to file a police complaint, inform your housing society, issue news paper notice and make application for duplicate copy of your registered document before the registration office citing your document registration number
It will all depend on the title of the builder to the land in question. Nationalised Banks are usually very cautious while dispossing such loan. However, financial institutions are aggressive in that connection.
You would have to file a suit against the land owner and also join the society as a party to such suit and seek reliefs on the basis of your title to the land in question. In such suit, you can also ask for reliefs relating to possession of the land etc.