I've had this happen a few times (iPhone 6, iOS8.0.2), but I've found that powering off my phone and restarting seems to restore the data. None of it is truly gone for me, it remains logged and listed, but it's almost like it cannot access the database of data that's stored and it gets a bit wonky but I'm not certain on that. Hopefully Apple will have it fixed for the next version or the app will be dead in the water.