I'm not sure how you can accidentally sacrifice something (MTGO misclick perhaps?) but No Rest for the Wicked has a one-shot effect. It will take action on every creature card that is in your graveyard at the moment that it resolves provided that they were put there from the battlefield this turn. It doesn't set up any effect that would apply to things that happen later.
I am not sure how you "accidentally" sacrifice it (unless it is a misclick on MTGO or something), but its effect applies at one time: when the ability resolves. Anything that goes to the graveyard after the ability resolves will not be affected by the ability. You will not get them back.