Just happened to me for the first time. I was in full panic mode until my wife calmly asked, "Did you try taking the battery our and putting it back in?" Her cooler head prevailed.
At the time of the shut-down, my battery was probably around 50%, and the uptime had been about 3 days. I'm going to try doing a full manual shutdown every 2 days and see if that helps at all.
But this does worry me. I've only had this bad boy for a week. I'd really hate if this were a common occurence.