The state you might find yourself in, if your mind awakens while your body is still asleep. While you are asleep your body is always paralyzed, to stop it from acting out your dreams and perhaps hurt you in the process. You are normally just not aware of it, since you are unconscious. When this malfunctions you get sleepwalkers.
Sleep paralysis may often be very scary indeed; to wake up and not being able to move at all could drive most people into panic. Often it is also accompanied with loud, roaring, screeching, hissing "sounds" and/or vibrations in your body. Feeling presences and hearing voices, often evil-sounding, is also common.
However, these things are illusory, the frightened mind´s interpretation of the unknown. As a matter of fact, sleep paralysis is an excellent springboard for the wonderful thing known as Out-of-Body-Experiences (OBEs).