Recent notifications can also be accessed by pulling a "Notification Center" down from the top of the screen.
Users who prefer the old notification system can keep it by choosing the appropriate option in the Settings menu. Users are able to sign in to Twitter directly from the Settings menu.
Reset All Settings Step 02: Now tap on ‘Reset All Settings’ button it may ask for device security code or password enter that then confirm again by tapping on ‘Reset All Settings’ button prompt.
Step 03: Your device will reboot with reseted settings to factory default system wide.
So I called up Apple Online Support Toll Free number what they said that this was happening because I installed new operating system via Wi Fi it may cause errors because of Wi Fi network loss or network interruptions.