Please help me!

My Macbook Pro had it's hard drive changed by apple when it suddently crashed in my house, the Macbook is one year old and has worked percectly since but when the people at apple changed my hard drive all my music on iTunes were erased, they didn't tell me why. Story short how can I transfer the music from my iPod where I have all my music to my iTunes? Please help, I dont want to loose all the money I spent on the iTunes store.