"First we were hit with an attack that would [reset] our DNS IP so that you could not connect," the company said. "This morning we are being hit, again, with a DDoS attack on our login server. We are implementing additional DDoS protection and hope to have this resolved asap. Unfortunately with these types of attacks it is very difficult to give a good time frame of when it will be resolved."
If you're still playing The War Z, there are further status updates to be found on the game's official forums.