At the moment, PlayStation Network is experiencing a bit of a hiccup, particularly in the UK and Europe. The reports are various, with some users claiming they simply can not access, while others say they are only running into minor network problems. The lists of friends are not loaded correctly, the PlayStation Store does not work, all that kind of stuff.
The same PlayStation has yet to comment, but if these reports continue to come, we are sure that it will only be a matter of time. We will keep you updated.
To update: Functionality seems to have been restored now. Fortunately, Sony seems to absorb these interruptions rather quickly these days.