If you were playing on iOS and suddenly find yourself in a new base instead of the one you were playing, or if you are showing up as "Null" in your Game Center Notifications we can help you with that!
Try this first; in game (on the new base you don't want to play) tap on your Avatar. Then, tap Settings, and select Account Management and Log Out.
Now, sign in with the credential you used to create the base you want to keep playing. If you don't know what credential provider you used, it is likely that you created the base automatically with Gamecenter the first time you opened VC. Once you sign back in, you should be good to go!
If for any reason you do not have your base back after this, you may want to try this additional step as well. First leave the game and go to you iOS settings. Scroll down to the Game Center section of your settings and then click on the Apple ID section at the top; click "Sign Out" from the pop up. Once you are signed out simply sign back in and you base should be back.