Hard to help without knowing what platform, but here are some general PC tips:
- Cities of Tomorrow is an addon to the full game. Make sure you also have the base SimCity game purchased and installed.
- Make sure Origin is fully updated before launching SimCity
- In Origin, right-click the SimCity icon and check the box that says "Disable Origin In-game"
- Run the game in Administrator mode (right-click the shortcut, choose "run as administrator")
- Be patient with the first run; it can take a long while to update the program for the first time. Disable anti-virus and anti-malware to make sure they're not interfering with updates.
- Make sure anti-virus, anti-malware, and firewalls are allowing the program to contact the server. Add Simcity.exe to the exception list for all of them. BitDefender has been mentioned as blocking SimCity many times; try disabling it when running SimCity.
- Delete all the files in the UserData folder.
If you have a desktop shortcut you can right click the icon and select PROPERTIES then delete all files in the User Data folder.
If you launch through Origin go to C:\Program Files (x86)\Origin Games\SimCity\SimCityUserData and delete all files in the User Data folder.
Good luck!