Why when I got my new computer now using windows 8. I lost all my playlists and my purchased song list. Im missing so much music....I downloaded Itunes onto the new computer, I logged into my account w/ my apple ID expecting all my stuff to be as it was

My issue is just with itunes....not my ipod... Ive been afraid to plug my ipod in to charge it incase it automatically sincs with the Itunes I have now on my new computer since this itunes is empty of my purchased songs and self made playlists. I dont want my ipod erased! I just dont understand why I cant get someone on the phone with their support service with out a serial number?! Itunes does not have a serial number that I can find. I just want my account to be as it was on my old computer. I very much dont want to lose all my music 😟