It felt like my brain and body were shutting down?

It sounds like one of those sleep paralysis related problems. Here's the deal: as you sleep, your body goes through periods of paralysis along with the different stages of sleep. Sometimes, especially if you are overtired or stressed out, you will proceed right into the stage that goes along with the paralysis as you are first falling asleep, and you are still conscious enough to actually feel it. Many people also wake up to this feeling. In both cases, most people say it is a very frightening experience that is usually accompanied by a sensation of dread or the feeling that something scary or evil is in the room with them. I am almost positive that this is what's going on with you.