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.
How to Reset a Frozen iPod or iPhone
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 reset a Frozen iPod or iPhone