This is a bug that seems to periodically happen to me at the most inconvenient time, and it's when apps just won't open when launched, even when I haven't touched the app in a week. This seems to be a bug with iOS's auto updates, from what I noticed, it happens when apps have recently downloaded the update but hasn't installed yet. Quite an annoying bug, because it needs a restart. And I hate restarting my device. It does sometimes fixes on its own, but that takes awhile. Just saying.