There is no cannon where this question is answered definitively; however, it's quite likely Krell, like most traitors, was executed. 

It should be noted that there is a possibility that he has sent to Rura Penthe (Klingon prison planet from Star Trek 6).  Given the fact that he was working with the Romulans, this is unlikely.  The Romulans are considered honorless by most Klingons so working with them would be likely be punished by death.