Regarding not getting possession of plot, builder is not ret

I booked a plot in shree Raj infra, Lucknow. Project name: Gomati Estate phase 3. Booking & agreement was done in 2014. Possession time was October 2016. But builder isn't giving possession. He got ticket in up election& preparing for that. He is clearly denying to my parents that I will not give plot nor money before election. Actually it was on monthly installment basis. I got a kind of blood cancer, so stopped some installments in between. I have given 50% amount till now. My question is: he failed to give timely possession, so should he give my complete as amount now? Or he can deduct 20% . And what steps should be take for returning my money back?