Hades doesn't tell any other gods about the missing helm of darkness because he knew that no one would help him retrieve it; Hades also didn't want his situation to be revealed to other gods, in order for him to keep his reputation as the god of the underworld. I can theorize this conclusion because in the novel the text reads, ""I have said nothing of the helm's disappearance," Hades snarled, "because I had no illusions that anyone on Olympus would offer me the slightest justice, the slightest help. I can ill afford for word to get out that my most powerful weapon of fear is missing. " (Riordan, 273) by this text, I can determine that hades uses fear as a main root of his power and reputation. so when hades had lost his most powerful weapon, Hades is alarmed that the gods will fear him no longer.