Happened on the 5 I have as an extra dev model. Restarting did nothing, but I did notice that my iCloud account within the main settings was blank. When I tried to log in, got an error, tried again and it worked and all my email accounts reappeared.
Also noticed the phone auto restarting yesterday. Happened twice. No similar issues with my 5s, so I'm guessing it is hardware related.
Bugs all reported.