My iPod 32 g starts up but freezes
It shows the apple logo the turns over into a tribal sign and that's it nothing happens
iPod touch
If there is any other sign shown than the spinning wheel, for example a skull, your firmware has been modified, causing the system freeze.
Unauthorized modification of iOS has been a major source of instability, disruption of services, and other issues
If your iPod has not been jailbroken, try to connect in recovery mode, explained here:
iOS: Unable to update or restore