When does it occur?

Sleep paralysis usually occurs while falling asleep, which is called hypnagogic or predormital sleep paralysis and If it happens while you are waking up, it's called hypnopompic or postdormital sleep paralysis. As it occurs while you are falling asleep or while waking up, you wont be able to move your body. In this case, the only thing you can do is look around or roll your eyes and you will sometimes see a shadows around you and the room that you are in will look exactly the same. While experiencing it, you'll also feel like something or somebody is on top of you and that's why you wont be able to move until its done.