7G Nano says no music, but iTunes disagrees

Hello and welcome to Apple Support Communities, gyandev.
If I understand your post correctly, iTunes shows music on your iPod nano but when you try to play music the iPod nano says there is no music downloaded. You've restarted the device multiple times and that hasn't helped. I'l be happy to work with you to get you back to your music.
First, let's go ahead and sync the iPod nano with your iTunes library. This article will give you the steps to do that:
Sync your iPhone, iPad, or iPod touch with iTunes on your computer using USB
Let us know if that resolves this issue for you.
Thanks for using the Apple Support Communities. Take care.

Hello gyandev.
Restoring the iPod nano to factory setting and then loading the music again is the best option at this point.

You can find the steps to restore the iPod nano here:
Restore your iPhone, iPad, or iPod to factory settings
Let us know if that resolves this issue for you.
Thanks for using the Apple Support Communities. Cheers.

gyandev wrote:

In fact, I don't sync my iPod with my iTunes library because I manage 3 iPods and 2 iPads from my Mac, and all of them have different stuff on them. So it's manual all the time:

As no one else has mentioned this, I will.

You can use Sync to manage several iPods with the same iTunes Library. iTunes will recognise different iPods (and iPads) and then use the previously set up Sync rules to synchronise only the required content for that device.

It can be made simpler by using Playlists to synchronise each device. For example, create a Playlist for iPod #1, another Playlist for iPod #2 and so on. Then use Sync selected Playlists etc. to put the required Playlist onto each device. I would have thought it would save you time and more crucially, when one of those devices needs a Restore or to be replaced, putting the required content onto the restored or new device would be a couple of simple clicks.

Once songs are on the device, you do not have to use the Playlists to play anything. You can play by any of the previous methods you are currently using. All the Playlist is doing is managing the content.