What to do the seller filed a case on us after 35 yeras

Hai Sir,My grandfather brought 2 acres land from some one before 30 years back. Owner of the property was women (Wife) and documents written by her husband.They both are staying in Chennai from last 35 years.My grandfather asked so many times for the registration and in that agreement document also its clearly mentioned about the money and also When ever My grandfather asked for the registration they need to come to Registration office and need to do the registration.All the mattar clearly mentioned in the agreement papers as well. But after that my grandfather getting sick and he died. Next my father doesn't have much knowledge about that and he tries to contact them but no reply from there end.7 years back my father left from house due to his personal and family tensions but till now we didn't found him any where.But suddenly the women (who made the agreement docs 35 years back) put the case upon my family, Like they didn't made the agreement and My grandfather occupied the property