How to get land ownership back when fraud done

Before 2002, my father had purchased a plot on the name of my grandmother. and after few years he had lost the documents because he is illiterate person.and other person would see his documents.. after few years the plot had been sold in 2006.. but plot owner had not sold it.. Today when I see ,it is on other's person name.. someone had done some fraud with us.. please suggest me what I should do to get my plot?