Unable to get any life even when 75% charge left

My Kindle Fire HD7 lost all volume one day; then the next day I couldn't turn it on at all - it had been on charge and was at 75% when I turned it off after last use. Now no glimmer of life at all. There are no obvious signs of what may have happened to cause this. It lives on my bedside cabinet where it is at no risk of damage.