Are you talking about the Nuketown 2025 24/7 playlist? If so, Treyarch removed it. You can continue to play the map in Chaos Moshpit and Custom Games.
If you're not talking about the 24/7 playlist, then check the Custom Games map list to see if it's there. If it's not there, try redownloading it from your download history:
Settings > Account > Download History