"Humans. They're so fragile. They'll never survive without their technology. No matter how crude. Take that away, and they'll devolve back to the animals they really are. And you can't save all of them." - Brainiac, to Clark Kent (Vessel)
Dark Thursday was a worldwide blackout on the planet Earth, caused by Brainiac's computer virus, which he used Lex's computer to spread. It resulted in widespread civil unrest and massive riots in major cities across the world.
Dark Thursday was caused when General Zod was released from the Phantom Zone, possessed the body of Lex Luthor, and released Brainiac's virus upon the world. The virus caused all of the world's electrical appliances to shut down as a way to welcome General Zod. Many were killed and damage to the cities ranged from mild to extensive.
The following morning, Zod/Lex used the hard drive from the Black Ship to temporarily restore power in order to hack into The Pentagon and arm military satellites that would initiate a pulse, wiping out all of humankind and reshaping Earth into a new Krypton. Fortunately, Clark returned from the Phantom Zone and destroyed the black box with his heat vision which prevented the disaster from occurring.
After the events, the world tried to rebuild. In Metropolis, Lex donates millions of his fortune, in guilt, to rebuild, including a masquerade ball fund-raiser with the All-American Rejects. Dark Thursday led billionaire playboy Oliver Queen, a bully of Lex's from Excelsior Academy, to leave his home in Star City to investigate the destructive force of Lex Luthor as the Green Arrow.