Try searching your hdd for one of the songs. If you get no results you may have deleted them by accident(it happens quite a bit). You will need a file recovery program to try to get them back.
This thread may help:
DELETING FILES FROM LIMEWIRE LIBRARY-DON"T
I don't know if you can do this but, if the songs are on your ipod can you upload them back onto you hdd? Just a thought, I don't have an ipod
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