Sony published a notice informing Sony Playstation Network users about potential unauthorized access to personal information.

Sony says it is possible that the passwords of a small percentage of PLAYSTATION Network users may have been changed through unauthorized access, making it possible to view users’ personal information and/or use the Wallet for the PLAYSTATION Store.
PLAYSTATION Network accounts do not display entire credit card numbers, so any unauthorized access to your PLAYSTATION Network account is very unlikely to compromise your credit card number.
So if you cannot log-in to the Playstation network, you have been hacked. I have just checked my Playstation accounts and everything works fine.

Sony has taken measures to rectify this issue and system security is restored.
More details in this notice on Playstation UK site and on the Sony Playstation Japan site (Japanese).