No it's not piracy.
If you own it it's not piracy. That's the whole confusion behind piracy.
Most companies will you tell you it's illegal, but I don't see why not if as long as you can prove you bought and own the game (broken disk, product key, receipt etc)
so i have to have proof? oh gawd...my dad/mom throws away anything that is broken >_<
what game is it? maybe you can order a new disk
I'd suggest just contacting the publishing company for an extra. Usually if you mail in your proof (i.e. CD key) then they'll send you an extra CD.