PlayStation Network is currently down, as indicated by the official Network Service Status website. Account management, login, online play, and more have been affected by the unplanned outage.
It is currently unclear if this is a planned maintenance period, although it seems unlikely given the timing. Although outages on PSN are common, this one appears more severe affecting all server-oriented systems of PS4, PS3, and PS Vita.
During the outage only single-player games are playable, and only those that don't rely on online connections for game-centric features. Thankfully, PSN had a period of moderate stability during the 2015 holiday season, indicating that Sony has made efforts to improve the quality of its service.
No statement has been issued by Sony. Stay tuned for more information.