Has this ruined my diet and make me put on pounds?

For the past few days I've been eating really healthily and today I've had: blueberry and raspberry smoothy (with low fat milk only) and a plain Omlette (1egg), chicken and low fat soft cheese spread sandwich (on wholemeal bread) but then I had one of my friends mini Hershey bars ( the bitesize,1 square things)...has this completely ****** up my eating? Will I put on pounds?