Ennervate is the countercurse to the Stunning Spell. It is used to bring Stupefied/stunned people back to consciousness. It is, however, of no use against unconsciousness induced by some potions. Harry Potter discovered this when he attempted to revive Hogwarts headmaster Albus Dumbledore while retrieving one of Voldemort's Horcruxes (a magical artifact containing part of a wizard's soul). In this case, water was the only substance that could revive Dumbledore. The spell Ennervate is also useless for resurrecting the deceased.
Some versions of the books list this spell as 'enervate', which means, contrary to what most people believe, "to weaken", and not "to energize".