There’s an ongoing maintenance until 5PM PT. As announced yesterday,
PlayStation Network will undergo maintenance tomorrow, September 21st. Maintenance is expected to start at just after 8 am Pacific and will last for several hours, until approximately 5 pm Pacific. During this time, users won’t be able to access the PlayStation Store, PlayStation Home or Account Management, and may encounter difficulty while signing in to PSN.
The PSN will go live in the next few hours, so just be patient for a moment. You can always check @PlayStation for any progress of the maintenance or check this page to see if the network is already up.