Kids fall and hit their heads all the time so I wouldn't assume the fall unlocked something, however I do think there could be some connection from TBIs to stuttering. Maybe not TBI specifically but some kind of trauma that could impact the throat or head. This is really something to ask a really good expert in stuttering (your PCP wouldn't know). We wouldn't know either. Does she seem bothered / frustrated by her stuttering? If not, I wouldn't over stress about it. You can give it some time and see. You could also see an SLP (who's an expert in stuttering).