What do you think Wiesel means when he says, "He [God] has been hanged here, on the gallows"?

In Night, Elie Wiesel recounts a nightmare of reality which is not merely his own experience as a Holocaust survivor, but is in some ways symbolic of all human suffering, especially that of other Holocaust victims.
The huge question which confronted the Jews in the Holocaust was: how could God let this happen? There is, of course, no explanation. To say that God is dead—that God is a victim too, executed by the Nazis, as it were—is Wiesel's metaphorical way of expressing the unthinkable. It is as if the scale of murder and brutality is such that religion has been replaced by a new, alternate-universe reality in which the existence of, or belief in, God has been canceled out and destroyed.
When, at the close of Night, Elie and the other survivors have been liberated, he is able to look in a mirror, and he observes that "A corpse stared back at me." Not only does God seem dead to a survivor, but the survivors have become the living dead. The fact that Wiesel and the countless others who had been through this did continue to live—that he became a great writer and was able to tell the truth so eloquently to the world—alludes to the possibility that if man can triumph over this, then perhaps God is still alive.