I got out my my iPhone this evening and it was dead. No response from it, not even when I plugged it in to the mains (which has worked before when the battery ran out of charge). Attaching to a PC via USB did not revive it either. Very mysterious, given that it was working just fine on the way home from work.
Fortunately, this post had the answer – hold down the Power button and Home (square) button until the Apple logo appears. And you have to believe – no point counting to ten and giving up, really believe and the iPhone will come back to life. Worked for me (after 14 seconds). This article has even more tips in case the first doesn’t work.
Some might say that any hardware that relies on the user to google an obscure combination of buttons to turn on their phone has a screw loose – but I’m just happy to have my phone back.