Turning the device off and on is sometimes called a soft reset. This is similar to restarting a desktop computer and is often one of the first steps in troubleshooting a device. In most cases, a soft reset won't affect data on the device unless the battery is very low.
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Press and hold the Power button.
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Tap Restart.
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Tap Restart again to confirm.
If unable to perform a soft reset, remove then re-insert the battery.