what caused my iphone to stop working?

i was using the ipod when the screen randomnly turned on its side and all that apeared was the time bar up the top the rest was black. i held down the power button to turn it off but i cant slide the bar across to actually turn it off. the screen will light up when i touch it but wont respond in any other way, and the volume/silent buttons on the side still work, right now im just letting it run out of battery... is there a reason for it freezing?
iPhone 3G 16gig with the latest update