Itunes songs mysteriously disappear!?

I had about 67 songs on my itunes library, my brother recently put about 35 more on. Today I went on itunes and many songs were gone the only ones that are there are the original 67 like the newest ones that were put on are gone. They are still on my ipod though. If I plug my ipod in will those songs reappear onto itunes library or will they get deleted from my ipod as well?