iPod 4th gen won't download music

I have gotten my iPod replaced by shop recently as battery wasn't lasting so I have not long noticed my music wasn't on iPod so I have tried going through iTunes on iPod and down pleading them and I have tried connecting to laptop both ways says unable to download this is music already purchased that I need to put back on but it not working box comes up saying unable to download and then under that says unable to download at the time then you can press done or retry and under song title it just says download error no error code and iam not stuck on how to get music to go on now