Recently (Thursday, May 22), we did some maintenance on the servers. For some of you, this may have reset your world to a vanilla world. If so, there are 2 things you can do.
1) Faster and preferred: Select one of the prebuilt worlds from the home page. 
It may not work the first time. If so, try at least once more.
2) Slower but may recover lost world: Post on this thread and I'll schedule a time to see if I can recover your old world.
