If this is the case and you are using mods please try the following:
- Right click Cities in Steam
- Click "Properties"
- Click "Set Launch Options"
- Paste in: --noWorkshop
What this does is disable all your mods and Steam workshop assets and if the game launches fine after this it means that one or more of your custom assets are causing the problem. The solution is to remove your mods or turn them on one by one to see which mod / asset it causing the issue and then either remove it or update it.