Disconnect the device from it's cord, if plugged in.
Press and hold your Sleep/Wake button (on the top) and Home button (on the front) together for around 10 seconds.
When the Apple logo appears, release both buttons.
Next, tap Settings
tap "Clear History and Website Data"
The issue should hopefully be resolved.
Both of those options when performed together should work but if not, you can try the following;
This will not remove any personal data but will reset iOS options back to current system defaults. You may have to re-enter passwords for Wi-Fi and other services.
On the device;
tap Reset All Settings
The device should eventually restart.
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