You could send them a message, something basic saying hello. If you get a response, could see if they have any questions about the game which may be why they are not progressing. If after about a week you have no response, I think it is fair to say they have become an inactive/ idle city.
The game has a system of purging inactive/ idle cities. From the time a player creates a city, they have 30 days to become active and reach a city score of 300. After 30 days, if a player has not reached the minimum city score, they will be purged out of the world map you are on.
If a city reaches the minimum city score and/ or has a diamond balance of 200 or more, becomes inactive/ idle, the purging process will not remove them unfortunately.