Started city now no cities load in zone

Not sure if this was already answered, but not going to go through 793 pages to find out so: I've started a city on a map with 8 cities, went to play another city on the same map, and it wouldn't load. Thought it might have been the chosen map so changed to another map with only 5 cities. Same thing's happened.
Anyone had this happen or know what the cause is? Single player mode.