This may be common knowledge, but i just thought I'd share this just in case it comes in handy for someone. Today, while someone was trying to call me, my iPhone turned off and wouldn't turn back on again. I tried all sorts of quirky little tricks but to no avail. Spoke to Apple and they advised to hold down the home screen button together with the button on the top of the phone for 20 seconds and it should come back on. Which it did!