If you've got a 6th or 7th generation nano, try this article for instructions on how to reset it.
Even though it doesn't look much like a desktop or laptop computer, the iPod nano is essentially a small computer. And, just like a regular computer, sometimes it needs to be restarted. If you've got a frozen iPod nano that isn't responding to clicks, it probably needs a restart the same way a desktop computer would. Luckily, it's easy to reset an iPod nano. Here's how.
NOTE: This works for the following models:
NOTE: First make sure that your iPod's hold button isn't on. This is the little switch at the top of the iPod nano that you can move to "lock" the iPod's buttons. When you lock the nano, it won't respond to clicks, which can make it appear to be frozen. You can tell that the hold button is on because you'll see a little orange area at the top of the iPod nano and a lock icon on its screen.
If you see either of these indicators, move the switch back and see if this fixes the problem. If it doesn't, continue on:
Slide the hold switch to the On position (so that the orange appears) and then move it back to Off.
At the same time, hold down both the Menu button on the clickwheel and the center button. Press them for 6-10 seconds. This should reset the iPod nano. You'll know the nano is restarting when the screen darkens and then the Apple logo appears.
If this doesn't work the first time, repeat the steps and try resetting the nano again.
If a second or third reset doesn't solve your problem, try plugging your iPod nano into a power source (your computer or a wall outlet) and let it charge. Then repeat the steps.
If after all this, your nano still doesn't work, you may have a bigger problem, and ought to contact Apple to get more help.