Maybe your device is temporary frozen and is unable to detect the headphones or reroute the sound to them. Try restarting the device or turning it off and back on, after a few seconds.
If the issue still persists, check out the other solutions.
The headphones jack needs to be clean so that the device can recognize the headphones. Clean the jack with a soft cloth and rubbing (isopropyl) alcohol and plug headphones in once the headphones jack gets dry.
Some headphones have an additional, spinning volume controller, aside from the device's one. So, if the volume is turned down on that controller, the headphones won't produce sound, regardless of the volume level of the device. If this is the case, all you have to do is to turn up the volume.
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