Please note that there are different islands in the game. Some of them become available as you progress in the story and some of them are open only for a short period of time. Here you can find more information about the different islands in the game.
Sometimes you may notice that an island that you were exploring just yesterday suddenly disappeared. There is a reason for this. Treasure Island and all the event islands are only available for a certain period of time. You can always see the remaining time on the icon with the island's tasks on the right side of the screen.
Once time is up, the island disappears. And what is very important, is that all the resources that you left on the island will disappear with it. So, please make sure you don't leave anything valuable there!