New computer doesn't have old itunes songs? What should I do?

My computer was stolen and I recently got it replaced, I went on itunes and signed in but all of my songs I had purchased weren't there, gift card money was, but there was no music, Am I missing a step or are the songs down the drain? I spent a lot of money on songs and I don't know what to do, Its a macbook and so was my old one same make same model, what should I do to get my songs back? Help!!