Amnesia fics that could fit, one S6, the others where, as I remember it, Buffy is happy enough without her memories.
Wiping The Slate Clean by ya_lublyu_tebya *I have this in word if you would like it drop me a pm with your email address
AU Season 6. The reversal of the Tabula Rasa spell doesn't quite go according to plan and our favourite slayer and vampire are left memoryless. Just how will no memory of the past change their relationship?When She Was Younger by kittiekat
Set S4. Buffy makes a wish to get back to being fifteen and unSlayery. A wish that will have dire consequences, not only for herself. What will this do to the SlayerVamp relationship, one can only wonder. You an' Me Against the World by sweetprincipale
Set immediately after Something Blue. A run in with commandos ends up modifying Spike and Buffy's memories and isolating the pair. Believing themselves to be estranged lovers, they try to reconcile and still beat the Initiative.