If you mean one of the city provinces (encounters are the tasks inside provinces that you are exploring) that you visit, the main hall may be under construction. During construction, you cannot 'help' the hall. If there is no other culture or builders hut that is available, you won't be able to 'help' that city at all. If it stays that way, it may mean the player quit (the building updates don't take effect if you're offline when they finish. They complete when you log back in the next time.). They have an automated method for removing (or not) these 'dead' cities, but it may take some time for you to see it, depending on your location on the map.