I battled kyurem and accidentally killed it because i didnt wanna waste my master ball and i didnt save.
http://pokemondb.net/pokebase/49478/if-you-ko-kyurem-can-you-have-him-come-back http://pokemondb.net/pokebase/59063/how-do-i-rebattle-kyruem-in-pokemon-white http://pokemondb.net/pokebase/52874/what-happens-when-i-defeat-kyurem-in-white
well sense you said you didnt save have you tryed going back to where kyrem was before it might be still there