From apple:
http://docs.info.apple.com/article.html?artnum=61705
<ol>
<li>Toggle the Hold switch on and off. (Slide it to Hold, then turn it off
again.)</li>
<li>Press and hold the Menu and Center (Select) buttons simultaneously until
the Apple logo appears, about 6 to 10 seconds. You may need to repeat this
step.</li>
</ol>