UPDATE: All worlds are up and running. RuneScape released the following statement on their Facebook page. "We're aware of the issues with the login servers and are looking into it now - we appreciate your patience and hope to have things working again shortly."
RuneScape servers all went offline with no warning due to a server blackout. Players became aware of the blackout while attempting to access citadels or log in from lobby couldn't, and started commenting in clan chats. Forum posts proliferated, as did comments on the facebook page.
The reasons for the Jan. 11, 2013, RuneScape server blackout are still not known. RuneScape players can load the game and access the lobby, and talk in clan and friends chat channels. Players in game can still continue playing, as long as they don't lag out or otherwise disconnect.
Server blackouts are fairly common, and range in degree of outage and coverage. They range from sporadic worlds being down to the entire site becoming inaccessible and last from minutes to several hours in length.
Many players initially reacted to the news from the lobby in angst, as ships waited in port, with time for sailing elapsing.