how can i get a right title of my land?

sir, i purchased a land in 2002. but builder given me possession of another land ,which was unknown to me.builder also make boundary wall and written my name on the wall.but now when I went to make a house for me, another person filed a suit against me , showing a sale deed of the land made in 2002 in his name,which was totally unknown to me.what should I do.i have also taken electric connection and bank loan on the property given on possession.