In SimCity (aka SimCity 5, the 2013 version), if your primary server is having issues or is being worked on, or if you just simply want to start over, you’ll need to switch to a different server.
To change servers, you need to be on the first load screen where you get the game updates.
You’ll see the Select Server button in the bottom left hand corner.
Choose a server that’s available on this screen and hit the Okay button. Note that any servers that you’ve claimed a region on will show a city icon to the right.