Sometimes, restarting the device is all you need, in order to fix the issue.
Maybe the flashlight lens is dirty to that extent that you are unable to see when the flashlight is turned on. Try cleaning the lens using a piece of fabric with some rubbing alcohol on it and once you do that, try turning on the flashlight.
Maybe you can't turn on the flashlight because the device is out of power, and in power saving mode. Plug the device in a wall charger and test the flashlight. If it works, then the power saving mode was the reason why the flashlight didn't turn on.
If you use a protective case to protect your mobile device from shocks, ensure that the case doesn't cover the flashlight, which might be the reason why the flashlight doesn't glow.
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