Here's the solutions I'm familiar with:
A: Go to your neighborhood, log out. Log back in.
B: Un-install the game. Restart your device. Install the game. DO NOT touch the screen other than to log in and accept the license agreement until ALL the updates have finished downloading. I know it says to tap the screen to start playing while the updates download but that is for new players or people who want to reset their game.
C: Contact Customer Support and ask to have your game rolled back to before the loss of your town. When asked to select a Topic chose "Missing content".
