608.2b If the spell or ability specifies targets, it checks whether the targets are still legal. A target that’s no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. The spell or ability is countered if all its targets, for every instance of the word “target,” are now illegal. If the spell or ability is not countered, it will resolve normally. However, if any of its targets are illegal, the part of the spell or ability’s effect for which it is an illegal target can’t perform any actions on that target or make that target perform any actions. The effect may still determine information about illegal targets, though, and other parts of the effect for which those targets are not illegal may still affect them.
Example: Aura Blast is a white instant that reads, “Destroy target enchantment. Draw a card.” If the enchantment isn’t a legal target during Aura Blast’s resolution (say, if it has gained protection from white or left the battlefield), then Aura Blast is countered. Its controller doesn’t draw a card.

Example: Plague Spores reads, “Destroy target nonblack creature and target land. They can’t be regenerated.” Suppose the same animated land is chosen both as the nonblack creature and as the land, and the color of the creature land is changed to black before Plague Spores resolves. Plagues Spores isn’t countered because the black creature land is still a legal target for the “target land” part of the spell. The “destroy target nonblack creature” part of the spell won’t affect that permanent, but the “destroy target land” part of the spell will still destroy it. It can’t be regenerated.

701.5. Counter

701.5a To counter a spell or ability means to cancel it, removing it from the stack. It doesn’t resolve and none of its effects occur. A countered spell is put into its owner’s graveyard.

701.5b The player who cast a countered spell or activated a countered ability doesn’t get a “refund” of any costs that were paid.