Q: nano 6th turns on for a few seconds then screen goes black and nothing. what is wrong with it....

At the moment I say as I've fixed, momentarily:

I made a downgrade to 1.0

The problem, gentlemen, is the upgrade from 1.1 and 1.2

It is a pity that I lost all the new features offered by the new firmware , just because Apple staff could not think to try this new update, on ipods with some airtime.

I hope Apple pull out a version 1.3 with the ability to choose the feature detection, and the user can disable it at will.

Did you already try to restore your iPod?

Restoring iPod to factory settings

Also check this article: iPod nano (6th generation): Hardware troubleshooting