If you pre-ordered or bought Black Ops 2 on the day of it released you were rewarded with access to Nuketown 2025 and a double XP weekend on the game's first week of release.
But now the weekend's over, everyone is in shock both double XP and Nuketown 2025 playlist are gone. However if you want to play Nuketown 2025 you can do so in custom game.
Treyarch game director David Vonderhaar confirmed over on Twitter where he tweeted the following messages:
Double XP weekend is official over. That means Nuketown 2025 / 24-7 is as well. I know. RIGHT? Don't kill the messenger. +
Nuketown 2025 / 24-7 will be back for special events. You can always play it with your friends in Custom Games.