I've had the same thing happen to me 3 or 4 times during my life. I just assumed I was blacking out. Twice it happened while I was standing up walking. Which was scary. Everytime it happened I could hear everything going on around me but I couldn't see, I couldn't move, I couldn't speak. It would only last maybe 20 or 30 seconds and then it would all start to come back. Honestly, I just consider it blacking out whether that is really what it is or not.