Reviving Spell

Ennervate is the countercurse to the Stunning Spell. It is used to bring stunned individuals back to consciousness. It will not work against certain kinds of Dark Magic, for instance, the effects of the Emerald Green Potion. Ennervate is also useless for resurrecting the deceased.

Harry Potter discovered the limitations of Ennervate when he attempted to revive Hogwarts School of Witchcraft and Wizardry Headmaster Albus Dumbledore while retrieving one of Lord Voldemort's Horcruxes. In this case, water was the only substance that could revive Dumbledore.

Behind the scenes
Some versions of the books list this spell as 'Enervate', which means "to weaken," and not "to energize".