did you change from mac to windows or vice versa?

either way, visit Free Software Downloads and Software Reviews - Download.com and search "podutil" this will allow you to take all of your songs from your ipod and have these songs on your computer (providing you have enough disc space). when you have copied all of the songs you want to keep on your ipod, onto your computer, allow itunes to erase all of your songs, then when it has finished, simply add your old and new songs to your ipod. hope this is what you were looking for