This can often happen if you reinstall or do a device transfer or clear things in your devices memory . This happens to me quite a lot and your buildings should eventually return .
The black squares indicate the images are being downloaded. Please wait for some more time, and it should automatically load. Or you could also clear data for the app to re-download all the assets in the game. Also make sure your device is connected to a strong internet source.
Customer support service by UserEcho