Warzone 2's DMZ mode added a brand-new map, Building 21, which you need a hard-to-find keycard to access, but after being live for only 24 hours, it's gone. And nobody really knows why, or what's going on.
There are theories, like that it's a Zombies-style ARG in the vein of old Treyarch, or that it was bugged and needs fixed, or that this was simply a teaser before its proper launch. Whatever the case, you can't get back in, as "Building 21 has entered lockdown after significant infiltration. Access cards are disabled, but will be operational within the week. Prepare accordingly."
Temporarily closing down the map to fix bugs is the most likely reason given this announcement's wording, as typically keycards are put into your inventory so that, when you die, you lose them and have to find a new one to re-enter the area. In this case, Building 21. But, for some reason, the keycard was being stored in players' stash, meaning that you could enter Building 21 an infinite number of times, hence "significant infiltration" (thanks, NME).
Right now, when selecting a map, Building 21 has a timer for its return, and it looks like it'll be back just for Christmas. It's launching again on December 24 at 5am GMT, though it remains to be seen if you'll only need to find the keycard once or if it will go back to being an inventory item that you can lose. If you can lose it, that'll mean hunting in Al Mazrah to find one each time before you get a chance to play.
How long it will be around this time is also unclear. You might be best making space for launch day just in case Building 21 gets yanked away again. Though, if old Warzone is anything to go by, it might simply be an indicator toward plans for map rotation, potentially locking off several maps while opening others, with Building 21 paving the way.
To get in, only the party leader needs a key, so if one of your friends manages to get a keycard, you'll all be able to dive into Building 21 together. It's described as "one of the deadliest areas of DMZ", containing a lot of "high-tier Contraband". In other words, its a big box full of loot waiting for you to dive in and rummage through. When it launched, the dev did say that the "window to strike is limited", which is where a lot of the ARG and planned theories come from, rather than the idea it was a bug that needed fixing. Whatever the case, sleuths will no doubt be back in there later this week to see if anything new has been added, or if anything vital has been changed.