My Ipod nano 3rd generation has lost all it's music and won't resync when I plug it into my IMac?

I have an Ipod Nano, and it somehow lost all it's stuff when I plugged it in at work to be updated. It won't sync the music back onto my ipod when I plug it in at home and it should just sync when I plug it in to my IMac. I have an Iphone 4G that sync's just fine. Not sure why...screen says connected when I plug it in, but then I remove it and no music or pix.

Help?