On rare occasions an iPhone or iPad can simply stop responding all together. No piece of tech is completely perfect. If you ever come into the situation of a frozen iPhone or iPad here is the procedure to get it to reboot.
First press and hold the sleep/wake button at the top, then right away, press and hold the home button. You'll need to hold the two buttons down for a while until the Apple logo appears. Your device should then be rebooting.
For those who are interested, this is the theme I'm using - Jaku.