You had found a way to summon the yellow eyed demon, making a deal with a demon, offering things up to get Dean's humanity back. You couldn't stand seeing him all evil-like, so you made your offers to which the devil declined, until you offered your humanity to return Dean's, Yellow eyes took the deal and so now he'd be back to his mortal, supernatural hunting self meanwhile, you went to hell and through the process Dean had gone through, your humanity being stripped from you, piece by piece.

Time passes, who knows how long it was, not you, and the torment did it's numbers, all for Dean. Except, dammit, Dean went looking for you.