What happened to Yachiru?
The very last page of Bleach #668 gives a few clues.

Yachiru suddenly appears out of nowhere and says to Kenpachi:


   Silly goose, if you would just use me properly, there wouldn't be anyone you wouldn't be able to cut down.


She then touches him, which causes:


   him to get a power boost, which he wonders about.


She ends the exchange with explaining to him:


   That power is what everyone's been calling "Bankai".


All of these suggest the following:


   Yachiru is Kenpachi's Bankai. He is unable to call out his Bankai anymore (he was never able to, consciously - the original appearance may have happened subconsciously), therefore, Yachiru is gone.


Alternatively:


   Yachiru is his Zanpaktou spirit, again, one he has never been able to consciously communicate with. Though, I can't explain her disappearance if that were the case.