My 5S has been powering itself off at odd times since the iOS 8 upgrade. It isn't a battery problem, as it had been as high as 47% charged.
On Wednesday evening it turned itself off at 27% and when I turned it on the Apple logo stayed on the screen. I went online when I got home and found that this is called "recovery mode".
I followed the instructions on Apple's website and it didn't work.
What to do. I had little luck with O2 online and ended up booking an Apple Genius.
Just in case I called into the O2 shop in Folkestone and they managed quite easily to reset it back to factory setup.
All that means I needed to was attach to my iTunes and restore my data.
After the first stage it looked a bit dodgy as it seemed to have lost all my apps and data associated with them. So I left it to sync again....