Today Sony have released an update via the Playstation Blog. The update states that it is an external intrusion which affect the Playstation Network and Qriocity Services (this is the music streaming service offered by Sony). The update gave no mention of when services will be restored.

Here is what has said via the Playstation blog in full:

An external intrusion on our system has affected our PlayStation Network and Qriocity services. In order to conduct a thorough investigation and to verify the smooth and secure operation of our network services going forward, we turned off PlayStation Network & Qriocity services on the evening of Wednesday, April 20th. Providing quality entertainment services to our customers and partners is our utmost priority. We are doing all we can to resolve this situation quickly, and we once again thank you for your patience. We will continue to update you promptly as we have additional information to share.

Source: Playstation Blog