You have my sympathy, Cathay Brat. Here are some suggestions:
1. As previously posted, you can download again all your iTune downloads from your account on the iTunes store.
2. If you downloaded from CD's then you can download again if you have the CD, or if you downloaded from a friends'.
3. If you have a list of your songs and you legally paid for them either via CD, iTunes or some other download system, you can search for them on Spotify if you have access to it (it's not available in the US, for example) and download them with a piece of software called Audio Hijack Pro. As I am a musician, I would only suggest this if you actually had purchased them. Unfortunately, however, many people do not have that sort of altruism and consequently musicians and artists lose their rightful royalties. If you are still stuck, send me a private message and I'll see what I can do.