Why couldn't I move my arm for a few seconds? Please help!?

Hey! The other night I was sleeping, and suddenly I woke up from bed. I couldn't feel my left arm..LITERALLY I couldn't even move it for 1cm! I helped my left arm raise with the other arm, and when I let go my left arm..it just fell-like it was dead!Then, around 8 seconds later, I started to feel coldness in my arm, and finally I managed to move it. Could someone tell me what happened exactly-why couldn't I move my full left arm for like a few seconds? Please I need serious answers because it just freaked me out and I'm worried. Thanks