Electrical arc problem
The breaker didn't trip because it didn't need to.  A bad connection failed first.  

Either in that junction box, or in the junction box before (next closer to the breaker box), you're going to find a scorched and arced wire on a screw or (far more likely) in a backstab.  This is why we don't like backstabs.  

Arc fault breakers will protect you from fires starting from arcing backstabs.