hard restart phone (power button + volume down button ) release when apple logo shows up
if that does work
reset all settings ( settings> general > reset > reset all settings ( you wont lose anything )
if that doesn't work
back up iPhone to iCloud or iTunes on computer pc,mac (per caution)
after backing up plug phone into computer that has newest iTunes
hold down power + volume down buttons till connect to iTunes screen shows up
message will come up in iTunes wanting you to restore or update choose UPDATE which will reinstall the ios and put new copy on Without deleting anything off iPhone if that doesn't work then we will do the restore option
and once phone is restored and your setting it back up FIRST thing is make sure we set it up as a new device
if issue is occurring go to apple store for genius appointment if not occurring restore there back up onto phone
and should be ok