Ghaziabad development authority claims property & sent notice

Please suggest I purchased a flat 5 years back as resale from a person. That person was the first owner of that flat. Age of building 12 years. I have also taken a home loan from a government bank. This property was first registered in 1972 in registrar office as housing property. After that two times land purchas,e a builder builds apartments. Now Ghaziabad development authority is claiming that this is her property/land and we have received Notice that within 15 days tell the reasons why we are staying there. On that property there are now approx 22 flats and 5 shops. None of us were aware about this. Most of the owners have taken home loan from govt banks. Even on the land the builder took a loan from govt bank. What should I do now? Religion: Hindu Thanks