Okay, I think I have posted a thingo looking for this fic before and someone found it for me but alas I have lost it again :(

It was a REALLY REALLY long Spander, and I think Spike gets amnesia or something, from stepping in a puddle before being electrocuted pretty badly from what I recall, and when he wakes up he's.... well pathetic really. And not Spike. So Xander nurses him back to health and I think there may be something about Spike becoming completely docile when called William, but that could be from another fic.

FOUND!! Thankyou emelye_miller!!
Lost by Velvetcrypt