Reset all settings.
Resetting all settings, as the name implies, resets all of your iPhone's settings to factory defaults, including your Wi-Fi, Bluetooth, Privacy, and other preferences. The settings will be reset, but all apps, email accounts, messages, and data will stay intact.
To reset all of your iPhone 6 Plus's settings, go to Settings->General->Reset->and then hit the Reset All Settings button. It's possible that you'll have to hit the button twice to confirm your action. Your iPhone will reboot once the operation is finished.
Uninstall any apps that aren't working properly.
If the issue arises after installing a third-party app on your iPhone running iOS 9.3, you can temporarily disable or delete the defective app(s) before rebooting your device. It's also conceivable that one of your recently installed apps is the problem.
So try to recall which apps you’ve recently installed before the onset of the problem.
