Authorization?? ipod help!!!?

Okay i downloaded new software andi"sync" or wat ever deleted all the songs but 15 of them, so i tried everthing to download those songs that i bought from tiunes back on to my ipod. And it keeps saying the same thing, I do not have authorizatoin to play this song on my computer. and it is really fustrating. My dad knows he didn't put a lock on it or anything. So then I "ynsync" it and it deleted ALL of mi songs. So, if anyone has an answer please reply back!!!!!