Elizabeth and William are lovers in the past. He gets poisoned by the same vampire poison that nearly killed Angel. She does a spell to get him back but the demon tricks her and she wakes up when Buffy becomes that old-timey girl in the episode Halloween. Problem is William is now Spike and doesn't remember her. I've searched everywhere for this fic but can't for the life of me remember the author or title. Please help.

That sounds like it could be Holly's fiction Echoes, which you won't find anywhere because it was turned into original fiction. You'd have to contact her for a copy, or maybe someone in here has one.

See, I thought it was by her but couldn't find it under her stories. Thank you.

Echoes is on the 'Do Not Exchange here' list, by the author's request. So moving this on to that thread