We noted an error earlier this evening which was causing the Lake Wintergrasp PvP zone to be reset prematurely. Since ending epic battles early is a bad thing, Blizzard has gone ahead and "shut off Wintergrasp" completely for the time being.
This change is affecting all realms.
There is no word when a fix will come through, but we're hopeful it will happen sometime soon. Disabling an entire part of the game is a big thing, and an error that usually is corrected quickly.
Some speculation points to the change in daily (now weekly) quests that just went in, however there is no evidence to support this.