Q: itunes won't transfer my music to ipod classic and deletes all the songs

Always had my ipod classic to manually transfer songs from itunes.  Have over 4000 songs on my ipod now.  Not on itunes.  Itunes is recognising my ipod  but every time I try to manually move the music across to the ipod nothing gets transferred and itunes won't locate the songs anymore (exclamation mark in circle next to each song on itunes).