WHY CAN'T I WAKE UP OR MOVE OR OPEN MY EYES?

this has happened to me on 3 seperate occasions.....i was layin down on my bed face down towards da window and then i felt like someone was walkin on top of my back but i could not move or talk n my body only moved down wen watever it was was walking on me..... then i felt it go out the window and about a minute later i could open my eyes and i could move again but while whatever was in my room at the time, i could not move or talk or even open my eyes....