I have experienced this problem of not being able to return to my city after watching a prize video (where you can win golden keys etc.). Normally when the video demo game has ended I click on the X icon to go back to my city, but yesterday I just got a white blank screen. DO NOT PANIC IF YOU HAVE CONNECTED YOUR CITY WITH FACEBOOK! The way I recovered my city was by uninstalling it on my Kindle Fire, and then installing a new SimCity Buildit game. Follow through the tutorial until you come to the stage of naming your new city. After doing that, you will see the option of logging in to Facebook, and this will bring up the name and data of your previous (lost) city that was connected with Facebook. Click continue to access your lost city and you are immediately back in to it.
I hope this will help those who feared their city had disappeared down a black hole as it is so soul destroying for a city you worked hard to develop (and as in my case used your own money to buy Sim cash), to then disappear.
video? You still get video? You lucky dog, my videos stopped about 2 million Sims ago, good luck.
