Occasionally an iPod or iPhone may freeze or fail to respond to your commands. This can happen for a number of reasons and does not necessarily indicate a serious problem. Sometimes they just need a restart much like the way your computer needs a reset once in a while.
Restarting an iPod or iPhone is a simple procedure that will not erase any of your music, data files, or customized preferences.
Tutorial: How to Restart a Frozen iPod or iPhone
Sometimes the iPhone just gets confused. For example, the iPhone may be on but it won’t respond to any button, switch, or touchscreen input. Try pressing and holding the Home button on the front of the iPhone for 6 seconds until the active program closes.
If this technique does not resolve the issue or the iPhone is in a different state such as showing a persistent Apple logo on the display, then you should reset the iPhone.