Sony has announced that the PlayStation Network (PSN) will be down for an update of the PlayStation Store on Monday, March 4th. That's today. The maintenance began at 5am PDT and end around 9PM PDT, so it'll be down for the whole day—or at least until Tomb Raider comes out later tonight.
During this maintenance period, you can expect to receive a message that reads "Sony Entertainment Network is undergoing maintenance" when attempting to log onto the PlayStation store.
Sony additionally states that if you logged onto PSN in the past 5 days, you may still have access to apps like Netflix and HULU Plus as your account will still be cached by the service. However, if you didn't log in during the past week, you'll have to wait until the maintenance period is over.
So if you're wondering why you can't connect online to play Call of Duty or what-have-you, don't fret—it'll be up. Eventually.