Option 2 did the job for me:
Press and hold the Sleep/Wake button and the Home button simultaneously for at least 10 seconds. The red slider may appear and the screen may dim but don not release the two buttons until the Apple logo appears
It is the second time now that the iPhone just dies. Nothing happens when I press the button. The first time it just started working again after a couple of hours, without me doing anything. But this time I did not have the time to wait, so after a short google search I found the article at Know Your Cell. Thanks.