Minecraft Pocket Edition World Problem

You are on Peaceful. Load your save, open the Pause menu, choose Options and from the first screen that appears (the Game options menu) slide the Difficulty slider all the way to the right.

There is no longer a "Peaceful mode" control, but there is still Peaceful mode. It is activated by setting the difficulty slider to the left. The difficulty slider now has only two settings: left and right, with no middle setting. Left is Peaceful, right is normal difficulty.

This is a quirk that you sometimes get on the App Store. Usually, you can just press buy again, and after you enter your password, it tells you something like:

You have already brought this product and will not be charged again.

However, if you don't want to risk it, you can either: check the "Purchased" tab, anything you download of the Purchased tab should be free, as you own it, or, you could download your purchases onto iTunes, and then sync them on to your device.