When you or another player leave your city, city investments travel with to the new city. If you entered a city that does not have as much invested, this can cause buildings to downgrade and content to lock. This may mean items such as resources or heroes may no longer be available.
Keep in mind that when you leave your city on the first day you join, the city will take 10% of your investment. This added feature is to prevent players from city hopping. Don’t worry, the percentage will go down by 1% each day you stay in the city and after 10 days, you can leave without having any of your investment be taken away!
Additionally, you will not need re-equip or recruit locked heroes. Previously hired workers will also not re-lock upon building downgrades.