Different comic versions had different explanations for how Krypton was destroyed. The two mainstream versions were currently the "self-destruction theory" and the "planet explosion theory."
The former roughly meant that, due to various social problems, Kryptonian society's development had nearly stagnated. The social issues brought about by rigid class divisions had accumulated for a long time before suddenly erupting at a certain point. A genetic disaster swept across the planet and wiped out civilization.
The latter version was much simpler. The Kryptonians had been living perfectly well on their own planet when something suddenly went wrong with their homeworld. It went "boom," exploded, and every Kryptonian died.
Some versions combined the two: social problems drove someone to blow up the planet, and the Kryptonians were wiped out.
But according to Green Lantern Hal's investigation, at least in their universe, Krypton had not exploded. The planet was still where it had always been; only the civilization on it was gone.
And because Kryptonian technology had been quite advanced, it had left behind plenty of traces throughout the universe. The destruction of this civilization could be traced, and it was not much of a secret.
After investigating for a while, Hal had more or less figured out how Krypton had been destroyed back then.
It could be called a social problem, but upon closer study, the real issue lay in their life form itself.
The Kryptonian race was not a natural race. In other words, they were not products of natural evolution. Every Kryptonian was born through strict Gene Screening, and their race's core technology was genetic modification.
That meant they had likely been as fragile as humans when they were first created. But after countless genetic modifications and rounds of screening, they had ultimately become such a powerful race.
It did