The question is simply too vague to answer adequately. Had the Normandy landing been repelled, it's unlikely the Allies would have tried to land in S. France for at least several more months. If it was turned back quickly enough, the Wehrmacht might have been able to transfer powerful armoured forces east. This could have turned the Soviet "Operation Bagration" into a fiasco & set its plans back considerably. It's doubtful that Germany could have created more than a "dirty bomb", but possible that the Allies may have been receptive to peace talks: The U.K. was reaching the end of its tether by mid-'44, & the U.S. might have been anxious to have an end of things in the Pacific. When you ask such a vague question, it's possible to answer with almost anything, & it can't really be disputed.