If you search the forum for it you will find there are numerous threads on sleep paralysis. I suffer from sleep paralysis myself. It's caused by the brain not turning on the neural pathways to the body upon waking, which are typically shut down so you won't move while you sleep and hurt yourself. Sleep paralysis happens when you become awake before your body does.

It can be a symptom of narcolepsy. It is not the same thing as being in a nightmare and not being able to wake up, which many make the mistake of associating. It's actually when you are already awake and cannot move your body.

I hear you on being afraid to fall asleep because of it. I have had many sleepless nights because of it.