Why is Ruth not speaking?
Ruth has been scared into silence by the bullying Abigail. She does so by making Ruth and the other girls complicit the story they makeup about witchcraft to cover up their illicit activities in the woods.
Abigail, afraid of her own punishment, growls to the girls in Act One:
Now look you. All of you. We danced. And Tituba conjured Ruth Putnam's dead sisters. And that is all. And mark this. Let either of you breathe a word, or the edge of a word, about the other things, and I will come to you in the black of some terrible night and I will bring you a pointy reckoning that will shudder you...I can make you wish you had never seen the sun go down!
Note here the explicit threat of "pointy reckoning," a threat of a knife and also an allusion to Satan's pitchfork. Young Ruth, terrified enough after the events in the woods, does not have the fortitude to stand up to the terrifying Abigail and retreats into silence for self-protection.