I've had my iPhone 4 for about a year. Suddenly today, it's started shutting down for no apparent reason. It happened twice earlier, and I was able to do a hard reset (holding the power and lock buttons for 10 secs), but now it won't turn on at all. It's now been off for about an hour, and even more disturbingly, it's quite warm to the touch even tho, I've just had it sitting here for a while! Anyone had similar issues or have ideas of how to fix?