Hey, Manuel
The issue is likely that your IPod's battery is improperly connected or just doesn't work properly. I would suggest making sure the battery is properly seated and all connections are proper. If it still doesn't work, try replacing the battery.
***Pages of Interest:***
[[Topic:iPod Nano 6th Generation|iPod Nano Gen 6 IFixit Page|new_window=true]]
[link|https://www.ifixit.com/Parts/iPod_Nano_6th_Generation/Batteries|iPod Nano Gen 6 Replacement Batteries|new_window=true]
[link|http://en.wikipedia.org/wiki/IPod_Nano#Sixth_generation|iPod Nano Gen 6 Wikipedia Page|new_window=true]
[link|https://support.apple.com/kb/sp593?locale=en_US|iPod Nano Technical Specifications|new_window=true]