Your City has not been reset – you are simply on another server. To resolve this issue, you need to:
1. Click on or open your "Settings" menu
2. Choose the server you used to play on and click OK
You can also access the “Switch Server” menu by clicking on the Server title on the top panel next to your City name. Players now have the opportunity to run 2 separate Cities on 2 different Servers simultaneously! If you are tired of your old City, you can simply switch servers and start an entirely new City in the new world - the choice is yours.