I had this happen to me a few days ago, and in that instance it happened twice in one day. I did a hard reset, as I was having another issue, and its probably coincidence but it hasn't happened since.
As much as I love this device when it's ON, I've never been so frustrated at were what I thought to be mundane things. C'mon MS, this is 2014. Putting your computer to sleep shouldn't be Russian roulette, now on top of that I got to worry about reattaching the cover.