Why did iTunes delete all the music off of my iPod??

I have two first gen Nanos hooked up to the computer to charge. One of them (4gig), mine, has been functioning fine and connected to this computer before. The other has never been hooked up to this computer. iTunes found it but repeatedly disconnected and connected it saying "update complete." Now, all music is gone off both iPods!!! No explanation, no errors, no pop-ups, nothing... music gone. Not a problem for mine because everything is on my computer. The other, however, is from another state, music can't be replaced. Why did it do this?

PC Windows XP