I have had that happen to me many times. Sometimes it is caused by my anxiety, like when I am in an interview, I try so hard to look like I am paying attention so I am busy focusing on nodding my head, looking interested, but then I realize that I had no idea what they just said. Other times I am sure it is my mind shutting itself down to hide from the pain or something...I have totally spaced out for minutes staring at a rock in the snow. It sometimes feels good to have absolutely nothing on the mind, especially at times when there is so much stuff going on in there...