The Germans lost the war in the winter of 1942-43 at Stalingrad (Volgograd) and the subsequent spring tank battle at Kursk.
You could easily argue that the Germans lost the war when they didn't capture Leningrad and Moscow in the first 6 months of Operation Barbarossa...however, the fact remains that the Russians took all the Germans could throw at them and, in the end, pretty much defeated them. In the Battle of Stalingrad, the Russians lost more soldiers than the Americans did in all of WWII...
Yes, the turning tide (Stalingrad) and the coup-de-grace of German offensive capability (Kursk).
The die might well have been cast on January 12, 1942 when General Von Reichenau died after a cross-country run in -20F weather. Rather than a strong, proven commander, the Sixth Army was led eastward that summer by the rather feckless von Paulus, who couldn't seem to bring himself to defy Hitler from inside the cauldron until he was already too weak to breakout. What the Sixth army needed was someone better equipped to stand up to Hitler, to make clear to him that logistic disasters could not be resolved with a wave of der Fuhrer's hand across the map, and that concessions and regroupments and yes retreats had to be made to save any of the Sixth army at all.
I highly recommend Anthony Beevor's excellent "Stalingrad."
The war was totally lost after Bagaration and the total destruction of Army Group Center.