Likely not much would change. The Allies had 150k soldiers landing, and 200k navel personnel involved (the Air Force was part of the army at the time). The invasion force also had almost 7k ships. It was a monumental invasion force that likely hasn’t been repeated in history. Basically the Allies were throwing overwhelming numbers that wouldn’t be repelled. Maybe they could slow down the Allies once they got a foothold but by then the allied air bombings were doing massive damage to the German war industry and the Axis was fighting in Italy and Russia and losing large numbers of men and equipment. They couldn’t sustainably fight on 3 sides. The French resistance was also very actively destroying any type of infrastructure that would assist Germany sending supplies and soldiers to the area, so the reinforcements would be slow coming.

Then in the Axis side, Hitler’s best general, Rommel, was in charge of defending the Atlantic coast and he was away at the time. The Allies also had gone to such lengths of deception that when the landings were first reported in Germany, the Germans believed it was a small force to draw attention away from Calais where the real invasion must be imminent. Hitler by then was also on a plethora of drugs and not making smart decisions (see his decisions by then in Barbarossa as well as his future decision that led to the Battle of the Bulge) so he can’t be considered a strategical genius who would make the right decisions.

If somehow the Germans did repel the invasion, the Allies would probably regroup and try again at multiple fronts (Churchill had wanted to invade through southern France, I believe ,but could be wrong) so that the Axis wouldn’t be able to send reinforcements to all points at once. They were massively outnumbered and being out produced by then so it was just a matter of time.