If it is in your Steam Library but not in the game, that's probably a Steam problem.
Please try a **clean** reinstall:
- Remove the game from Steam (uninstall)
- Manually delete all these files and folders, if still there:
...\Steam\steamapps\common\Cities Skylines\
...\Steam\SteamApps\workshop\content\255710\
...\AppData\Local\Colossal Order\Cities Skylines
(first move the /saves/ folder elsewhere to keep your save games)
(To see the AppData folder you'll need to enable View Hidden Items in File Explorer)
- Install the game again and see if it runs now.
Good luck with that!