well ... riot has never told us because amumu died and less because revived

They never told us because they want it to be a mystery.

I think after he died there was a timer for when he was supposed to wake back up again.
A timer that would go off when he had completely forgotten everything about his past life.
When the timer went off, runes would activate and he would wake back up.

I'd go with cursed tomb. When whomever found his tomb they awakened the curse and AMUMU with it. That is how i like to think of it.