>In all games, being defeated by a Legendary Pokémon does not cause it to disappear.
>In Pokémon FireRed, LeafGreen, and Emerald, if the player flees from a stationary Legendary Pokémon it will reappear upon leaving and re-entering the room, unlike the previous games where a stationary Legendary Pokémon could only be encountered once.
>In Pokémon Platinum, if the player defeats or flees from a stationary Legendary Pokémon, it reappears after the player enters the Hall of Fame again.
>Regirock, Regice, and Registeel will also reappear in their respective caves after entering the Hall of Fame if they are defeated or fled from.
It depends on the game. If you defeated Regirock in Platinum, Black 2/White 2, or ORAS you just need to beat the Elite Four and Champion and Regirock should return. If you were defeated by it in any of the game it appears, it should return after beating the Elite Four and Champion. If you ran from Regirock in any of the games it appears, it should reappear if you (In Emerald, FireRed, and LeafGreen) if you exit and reenter the room, or if you (In Platinum, Black 2, White 2, OR, and AS) beat the Elite Four and Champion.
There is a chance that I'm misinterpreting this info, so please comment if you think I am.