"The security break at Sony Pictures marks the second time that Sony Corporation had been targeted by hackers. In 2011, the online network for Sony's PlayStation game console was broken into, exposing names and credit card numbers for millions of customers. By the time damages from more than 50 class-action lawsuits had been paid, it's estimated that Sony spent more than $2 billion as a result of the breach.
"Further disturbing is that thus far the studio's IT experts have been unable to reverse the attack and get the computer system back to normal. “The IT department has absolutely no idea what hit them or if they can recover any of their files or operating systems, or even turn on their computers Monday,” said the insider. http://www.thewrap.com/sony-execs-working-on-chalkboards-while-hackers-claim-stolen-data-includes-stars-ids-budget-and-contract-figures/
and here you will find a good overview of what is lost (private keys, code, ID's, contracts,.....) and all other information you may need inside the discussion https://www.reddit.com/r/hacking/comments/2n9zhv/i_used_to_work_for_sony_pictures_my_friend_still