The War Z's community manager has gone to the official forums to let players know that the game servers have come under attack by unknown people or groups in an attempt to shut them down. The first attack occurred three days ago, the second earlier today.
Guys over the last few days we have been under attack by various forums of malicious attempts to shut down our servers. First we were hit with an attack that would rest our DNS IP so that you could not connect. This was solved a couple days ago early in the morning but took time to actually resolve world wide once the fix went in.
This morning we are being hit, again, with a DDOS attack on our login server. We are implementing additional counter attack solutions and how 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.
Check out the full thread on The War Z's forum.