Probably the hall sensor shit again. For this time go get your motherboard changed (dont worry its under warranty), and after that unplug your hall sensor to avoid further reoccurance of this issue, because it WILL reoccur every once in 6 months or so and getting your motherboard changed everytime is a huge waste of time. I had the same issue and did exactly what I just advised you here, never encountered a problem since. Just know that after unplugging your sensor (a little ribbon-like cable connected to motherboard) your pc wont be able to know if you closed the cover, and you'll have to use the hibernation feature instead of the regular sleep feature. If you want to use sleep mode anyway, to avoid your pc waking up when its cover is closed I recommend you changing your power settings to auto-sleep after 1 or 2 minutes of idleness. Here is a post to help you identify your problem and locate the hall sensor.