Any way to get back itunes music after losing it?

I upgrading my version of windows and I forgot to backup my iTunes music, is there a way I can re-download it from iTunes without buying it all again?

Send Apple a support email to the iTunes Store (e.g. by using this form). Tell them what happened and they should allow you to redownload all of your old purchases. That's what they did for me. You won't be able to redownload content that has changed on the store. It seems as though some songs get removed and then readded so their IDs won't match the same ones you purchased. I was able to redownload about 95% of my purchases.

Regardless of whether or not you agree with their policy of not allowing you to redownload purchased content; please don't abuse their generosity, and try to back it up yourself next time.

No.

Send them an email about your sob story, you never know.

(Lesson learned. Backup always. Especially before upgrades.)