I recommend following these steps if the screen on your iPod nano remains dark:
Will not turn on or screen remains dark
- Connect iPod to power for at least ten minutes to ensure that the battery has enough charge to allow iPod to turn on.
- If iPod does not turn on after ten minutes, try resetting the device while it is still connected to power. Reset the iPod by holding down the Sleep/Wake button and the Volume Down button for at least eight seconds or until you see the Apple logo appear.
- If the Apple logo appears on the display, connect iPod to a computer and verify that it appears in iTunes and can play music. If iPod appears in iTunes and can play music, no further troubleshooting is needed.
- If the screen remains dark and will not turn on, then your iPod may need service.
You can find the full article here (the title says iPod nano (6th generation) but the steps are applicable to the 7th generation iPod nano as well):
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