We can ask if Germany lost the war even earlier, for example when it failed to defeat Great Britain with airplanes and submarines, leaving it as an essential future base for massive US forces and a second front. Or when it just began its invasion of Russia. The answer to that is negative. As long as he wasn't at war with Russia, Hitler had options and possibilities, nothing was final yet. When he invaded Russia, he could still do things differently, such as concentrating the effort on Moscow from the beginning, and presenting the war as campaign of liberation from Stalin's brutal regime in order to soften russia's resistance, but Hitler interfered with the military conduct of the invasion from the beginning, and the unprecedented nazi brutality that aimed to decimate and enslave them, left the tough russian people with no other choice but to fight their toughest war, and utilize their endless resources much better than ever, and by doing so Hitler lost his last remaining options and his chance of winning the war.
So in December of 1941, at the gates of Moscow, Hitler's war was lost. It took 3 1/2 more years to end, thanks to the outstanding fighting skill and loyalty of the german soldier, but he could no longer win it.
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