What would have happened is anyone's guess. With the resources of continental Europe and Russia under its command, a Eurasia-spanning Third Reich would be very difficult to defeat. That's the difficult part about changing history. Have you read the novel Fatherland or watched the HBO TV-movie?
There is no guess... the moment the Nazi party got elected, Germany entered a downfall. Prior, German innovation was astounding in industry and military capacities. The brain-drain that resulted from the Nazi government that came to power greatly diminished it's ability to innovate - they had built up a great deal of technology innovation prior, but beyond 1939 tech left them hobbled even before they rolled into Poland. Airplanes like the 262, Fw-190, jet engines, tanks, u-Boats, the mighty Bismark, and even the V-1 rockets - they were ALL on someones drawing board prior to WWII starting. The pace the German Wehrmach and Luftwaffe advanced their designs, weapons, and technology was at a snails pace compared to American, Canadian, and British designers.
Don't get me wrong and say that Germany didn't advance their own tech, but the pace that the Allied forces were able to innovate their technology was hobbled by idealism, misguided direction from leadership, and their inability to accept that the Americans would ever enter the war.