Betty and Ruth both suffer from severe fright as a result of being caught in the woods dancing, chanting, and casting spells with Tituba and the other girls.
For one thing, the woods are considered evil and the dwelling place of the devil and his followers. We all know this is the only place the girls could carry out their harmless activities in private, but it is their bad luck that Reverend Parris happened along.
Also, Parris is her father. He is already on the outside of the community's heart by making unreasonable demands and requests of these people. She recognizes that her behavior would give the community one more reason to kick him to the curb and get themselves another reverend.
So, Betty's problem is that she fears her father's wrath as well as the community's judgement for her actions. Therefore, she is "frozen" and unable to wake up for a time being which is a defense mechanism employed by the girl's subconcious.