Well, I wasn't being stabbed, but I remember a time when I was around 10 years old where I was completely paralyzed by fear. I was lying in bed and was in deep REM sleep when I suddenly woke up and an image of a person that I was dreaming about was still being projected in my mind. This image, which was of a person dressed like a skeleton, walked in front of, and out of my open bedroom door. Being 10 years old, I saw it as someone in my room, and I tried to let out a scream, but nothing would come out. I tried to move to the other side of the bed, but I was completely frozen. Not one muscle would work. I also felt my heartbeat increase from a very slow rate to complete rapid fire in a matter of a couple of seconds.
This sounds like Sleep Paralysis, some more information here:
http://en.wikipedia.org/wiki/Sleep_paralysis
Interesting, but I don't think it was that. After about a minute, when I realized it was just a dream, I was fine and walked downstairs.