1. You mentioned "Flat was purchased from the cooperative society against payment as per payment schedule",
2. Did you make the payment to the Co-operative Society? If so, then how did the builder get his consideration?
3. However, if the Committee of the Society has become defunct, then call for a special general meeting by all the members of the society & select/elect the managing committee,
4. Conduct the meeting properly with quorum and record the proceeding and the resolution adopted by the meeting,
5. Once the new committe is formed, the problems can be sorted out.