EA and developer Maxis have announced that the SimCity servers are currently offline due to server maintenance. No game changes are expected at this time.
Take a look at the official EA announcement below:
Whilst the game servers are down for maintenance, Single-Player will be available as usual to all SimCity players, simply select Single-Player from the main menu to play offline.
At approximately 9:30am PDT/16:30 UTC/6:30pm CEST there will also be a small update to Installer files via Origin for PC players. This is a small update that all players will get but will only be utilised if you re-install SimCity. This update will apply in the background or will be downloaded with any new installation.
The SimCity server maintenance is expected to last several hours. We’ll update you once they are up and running again.
Source: EA Forums