Could Germany have won the war?
Surely by June 1944 it was too late, regardless of the outcome of D-Day. Only if repulsing the Normandy landings gained the Germans enough time to develop their "wonder weapons" including the A-Bomb (which they do in the book, but in reality they were no where near building) could they have perhaps dragged the war on for another year. Because the Soviets would still be on their offensive in the East, and the strategic bombing would still have continued from England. And when the Americans finally got the A-bomb in July 1945 and had it been used on German cities, I’m sure the Germans would have surrendered at once like the Japanese did.
Surely the Germans only chance to win was during the Battle of Britain in 1940. Because at the time, they were not fighting the USSR, and had they won that, and successfully invaded Britain, it would all have been over.
In the end Hitler lost the war by:
Not winning the Battle of Britain
Invading the USSR and fighting a war on two fronts
Declaring war on America after Pearl Harbor causing the onslaught of strategic bombing.