I just bought a used IPhone yesterday and everything was working fine. Today it went into sleep mode with 35% battery remaining. An hour later I pressed the home button and nothing happened. Then I tried the hold button and nothing. I plugged in the charger cord - nothing. I connected the USB cable to my computer - nothing. As a last resort I pressed and held the hold and home buttons at the same time. The phone powered up (Apple logo) and everything was fine. Any ideas on what happened? Is this a sign of an impending failure? Thanks for your help.