If D-Day failed, the Germans would have still lost the war by mid-45. The Soviet 1944 Summer Offensive alone nearly destroyed the German army in the East. The Western Allies would likely have tried another landing within months somewhere in Europe. Aside from that, the Germans were still engaged in Italy already. By this point in the war there was no German "heavy offensive" anywhere. The best they could do was conduct local counterattacks hoping for a bit of breathing space.
No, if the Germans were to win WW 2 they needed to stop after France and concentrate on getting a peace or victory over Britain. Involving the Soviets and then the US doomed them.