If you are trying launching the game during the maintenance break, you will see a maintenance notice at the game loading page with an estimation on when it will finish. 

While we understand that this prompt isn't what you'd like to see when you want to be developing your city or starting an attack, just like every computer system, upgrades and maintenances are necessary once in a while in order to keep things running smoothly and ensure the best game play experience possible.

As much as we want to keep the maintenance short and bring the servers back online as planned, there could always be unexpected situations. When that happens, please be assured that we won't stop working until we bring all of our servers back!