Because there was an attack of the PlayStation network, compromising the security of the site and its users. The network may still be offline for maintenance at various times.
Service was restored to most users on May 14, 2011 after the outage of April 20, 2011. Users were required to reset their passwords and download new software to connect to the network.
(to check the network status, see the related link)
The network is down more information is available at related question
The last item in the network to come back up was the PlayStation store on June 2 2011 after the April 20 2011 shutdown
The network was shutdown because it was attacked. The related question will be updated as new information is available
The PlayStation Store came back on June 2 2011 and was the last part of the PSN to come back online after the April 20 2011 shutdown
No the network is still down. The related question will be updated with the lasest status
No the network is still down. The related question will be updated with current information
The network will be down for days and the related question will be updated with the latest information
The network has been turned off on April 20 2011 because of an intrusion. The related question will have the latest information and the source as a link
No the system is still shutdown and the related question will be answered if new information is avaiable The PlayStation Network is still down they will working on it around the clock and they will be giving everyone who is a customer of the PSN 30 days free of playstation network premium.
It is not known when the network will be up, they are working on it. The related question will have the latest information
It appear that it will be down for a while yet The related question will be updated with the latest available information
The network has been signed off see related question for more information