Yesterday my iphone had about 85% battery level when it just switched itself off and became very warm for about 7 hours. The screen went black. I couldn't access the battery level sign, no icons, nothing! I couldn't switch it on again. Then after about 7 hours it finally became cool and displayed a red battery sign - i.e. very low battery. I recharged it and today it seems fine. Does anybody know what on earth happened and how I can prevent it occurring again?