Builder not provide property and proper response

I paid some amount to a builder account for a flat and he give me a notery as a document and he promised after one month we will complete all process all the registration process and hand over flat key within a year, after 20 day he died and his wife and all family members sayed we will do the work as per his promise but after one year they are still not starting the works and didn't give a proper response, please suggest what should what can i do?