PSN has been shut down for scheduled maintenance today. The blackout, which effects North America and Europe, started at 11am Eastern and will last until 2am. That's 8-11 Pacific, or 16:00-7:00 in the UK.
For many PS3 and Vita players, the mainatinance will not effect service at all: Anyone whose logged into PSN in the last five days will have be able to play games online and use apps. Players who haven't will be locked out with the message; "Playstation Network is undergoing maintenance."
Sony will announce via Twitter when they've finished up and everyone can log back in.