Not being able to speak

Date: 11/29/2017

Had a dream that I was in a car with one of my good friends and he fell asleep while driving. I remember trying to shake him as hard as I could to wake up, I tired yelling and screaming and nothing would come out. Couldn’t get him to wake up for the life of me. I woke up to my aunt trying to get me to wake up because I was screaming and crying in my sleep. What does this mean and why does it constantly keep happening????