Missing Music

Hello. I got a new computer and I logged into my iTunes account and 90% of my song are gone and I want to know why this has happened once again. This is not the first time this has happened and it makes me very upset and I'm not about to buy all the songs again that is way to much money that I have put in on buying these songs ive had this account for quite some time so it frustrates me that my music is gone. The last time this happened I was told to rebuy them.

Where did they go?

Where can I go to get my music back again?