I had this happen once and in my case it was definitely heat related. I had to leave my phone in the car at a golf tournament and the car interior got really hot. It wasn't in the direct sun, but it was hot to the touch. It had a full charge when I left it, but it would not turn on or respond to reset. I thought I had fried it, but after pulling and re-inserting the battery then leaving on the charger for 5-10 minutes it finally revived. Whew!