Sony will be conducting some maintenance on their dedicated PSN servers that should last around two hours starting on Monday, April 18th at 9:30PM PST/
During this maintenance period, you won't have access to account management, your PSN profile, PlayStation Video or even the PlayStation Store. All is not lost, however, because some basic, which means gaming, functionality will still be available. "Anyone who already has an account can still sign in to their PlayStation Network profile, play games, and use most applications while this maintenance is carried out," Sony explained on their maintenance site.
Such deliberate and invasive maintenance is necessary when upgrading their servers or applying larger, more sophisticated software patches that can't be done in the usual stages that wouldn't otherwise disrupt availability of services. This period is short, though it could go on longer than the projected 2 hours. It's certainly nice, for a change, to see maintenance scheduled and on purpose.