Catch thrown over boundary

What happens if a catch is taken by the boundary; the ball is throw up in celebration and goes over the boundary ...asks Sivagar Siva

Once a catch is complete (the catch is taken cleanly taken and the fielder is in control of his body and his movements), the batsman is out and the ball automatically becomes dead, so nothing changes; i.e. the batsman is still out (and it's not a 6). The umpire would be the sole judge of whether the catch was taken and the fielder was in control or not.