Until recently I was able to download music to my 3rd generation IPOD.
For some reason, the IPOD has disappeared from the ITunes device manager.
Now, not only can I not receive new music, I can't even find the device!
Any suggestions would be greatly appreciated.

I don't use Itunes, Ipod, or Mac OS. (sounds like the wizard in the Wizard of OZ )

But what I do have is

iPod troubleshooting basics and service FAQ

maybe you have to uninstall the orginal itunes version and install the newest one