Sounds like you dissociated. When you feel like that - can't connect, not entirely present, ..., that can be a chance to find out what feels so unsafe or scary, and address it. You can go back and work on this at your next session now that you are able to think about what happened. What was the last thing that you were able to think about or focus on? You can trace the feeling back to what started it. Your therapist can help you with that too, and also teach you how to use grounding skills to bring yourself back. Even though right now it might feel like a wasted session, if you are willing to talk to your T about what happened and work through it, this can be a chance to make progress.