What happened to Sakura?
Fate/hollow ataraxia is the official sequel to Fate/stay night. It has radically different feeling to it, bringing more comical relief and tongue-in-cheek references, but it's as close to "canon" as we can get. It doesn't follow any particular route from F/SN.

I didn't read F/HA, but here's how TYPE-MOON Wiki references Sakura's development:


  In Fate/hollow ataraxia, Sakura lives a peaceful life in Fuyuki City,
  which Rin has left in Sakura's care while she is at the Mage's
  Association, and Rider comments that Sakura is now the popular new
  captain of the school archery club. Sakura's dark side still exists,
  but appears only for comic relief. Shinji reveals to Shirou that
  Sakura assumed control of the Matou household, and both he and Zouken
  are terrified of her.
  
  In the eclipse story The backside of Kibisis Rider and Sakura seem to
  work together to seduce Shirou and engage in a threesome with him.
  Actually this is all Rider's doing, trying to get Shirou to pursue
  Sakura more aggressively. Yet in the middle, Shirou realizes the truth
  and reverses her spell on her, in the end revealing to viewers that
  Rider herself has deep feelings for Shirou. Sakura appearing in this
  episode is only a conjecture by Rider's spell, a representation of
  Rider's true intentions of her actions.


There doesn't seem to be any canonical explanation beyond this. Personally, I think it's part of why people like Heaven's Feel so much: the thought of Sakura undergoing torments under Zouken until she dies just doesn't feel right.