Board Thread:Questions and Answers/@comment-31578527-20170322201517/@comment-28905095-20170322211706

Have you tried to go into flight mode, and then starting MCPE? That could help in some cases, but not all.

One rather extreme variant, is to use iFunbox (iExplore) to make a backup of all your worlds, and then reinstall MCPE. After that, you could re-add the various worlds, making a note on which are playable or not. If lucky, all the worlds are playable again, and if not you might need to do another reinstall and add the other words.

After adding the worlds, you could try to re-add any texture packs, resource packs or behavior packs, you had before it crashed.

I've had to do this once, and it wasn't any fun, but it did work.