The lost slayer - Christopher Golden

In a single moment of bad judgement, Buffy finds herself catapulted into an alternate future dimension, where evil triumphs and vampires rule supreme. She discovers that her friends have rallied to fight a rearguard action against the new order - but they are very different from the friends she thought she knew. Willow is a fully-fledged sorceress, Xander a battle-scarred, humourless veteran and Oz a frightening split personality.
But that's not the worst of it. Her enemies are different too. That in itself is no surprise, but Buffy is horrified to learn that even the vicious Spike is merely a minion, chief lackey to the bloodsuckers' overlord, serving none other than Giles the Vampire King. Buffy must face and conquer the dark spectre of her erstwhile mentor, while her friends back in her own time struggle to bring her disembodied spirit home.
Imprisoned in the body of her 24-year-old self, Buffy's only hope is to uncover her past misstep and make it right...