Place the iOS device in Recovery Mode and then connect to your computer and restore via iTunes. The iPod will be erased.
If recovery mode does not work try DFU mode.
For how to restore:
To restore from backup see:
If you restore from iCloud backup the apps will be automatically downloaded. If you restore from iTunes backup the apps and music have to be in the iTunes library since synced media like apps and music are not included in the backup of the iOS device that iTunes makes.
You can redownload most iTunes purchases by:
If problem what happens or does not happen and when in the instructions? When you successfully get the iPod in recovery mode and connect to computer iTunes should say it found an iPod in recovery mode.
If you previously synced to the computer then you may be able to recover use of the iPod without erasing the iPod by following the instructions here:
I do not believe there's any way to recover from this other than to erase the iPod.
Was this music downloaded from the iTunes Store? If so, note that you can re-download it:
If it was purchased elsewhere, from a source that does not allow re-downloads, and you don't have any backups of that music, I'm afraid it's gone.