either
a. turn of the game, without saving and find him once more.
b. re-beat the elite four to make him come around again.
both options will make Tornadus reappear
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It goes away forever! Unless you trade for it...
Once you kill Tornadus, you have two chances. Since it is a legendary Pokemon, there is only one of its kind, and will appear after the first one.
If Cresselia faints [same as any legendary pokemon] you cannot catch it ever unless you save before the battle and restart if you make a mistake.
no because tornadus is a legendary srry
Provided Tornadus faints during a battle and you've saved after fainting it, it will respawn once you beat the Pokémon Champion. N will act as the Pokémon League Champion during the first challenge of the Elite 4 until you beat them, him and Ghetsis as well as capture Reshiram.