minecraft worlds disappear from the "singleplayer" world selection,but appears in the C:\Users\*username*\AppData\Roaming\.minecraft\saves

When I was making a city world(that I just started).But then, when I exited the world and pressed the "quit game" button, I was expecting(not really)that the world was still there. What I got was two worlds not there,but found them in the C:\Users*username*\AppData\Roaming\.minecraft\saves (Their names are "World" and "New World").