Iphone simply switched off mid-use

I was actually using safari to browse the Apple Store - was in the market for an ipad! I am now in a complete stress though, because while I was browsing (at about 15% battery) the phone simply switched off (as if the battery had died). I went to plug it in and while I pottered about, I left it charging in the wall. After about 10 minutes, the phone still hadn't switched on which I thought odd. There was no charging screen either so it was like the phone had simply died. I used my boyfriend's iphone to look up some ways of getting the **** thing working again... tried holding the home/off button simultaneously for 10 secs, 20 secs and 30 secs. Nothing. Tried plugging it into itunes - nothing, the screen stayed black. Tried plugging the USB into the laptop, holding the home button and then connecting the USB into the phone....nothing! I am getting really frustrated!! I am sure I got the phone under a year ago so I cannot think what has happened... its so strange. Any suggestions? Has this happened to anyone else? My provider is o2 - for half a minute I thought it was something to do with them again. PLease let it be their fault!! Any suggestions would be fantastic!