First Time Buyer- Property Issue

We are a First Time Buyer.. spent all retirement and hard earned money on buying a property by additionally applying a 45L home loan for the same... Now, All suddenly from nowhere some other XXX person claiming it's his property.. We tried calling the broker so many times but no response.. somehow found and reached to the property owner who sold it to us and he is claiming it's his property only no issues you start construction and I'll not involve in this as I sold it to you already.. Now, I'm struggling to build a home as the XXX person is bringing gundas and using filthy languages.. We have filed a complaint but XXX is not ready to come police station... Also, we have cross checked all the property details which is with us is up-to-date.. and the one which XXX is having is not proper documents... I'm 69+ retired old man.. Don't have strength to fight or roam to court.. I'm totally blind and hoping someone will help us for all my good deeds done throughout my life... Kindly do the needful.