What happens if you try a hard reset of your device? Does that help? To reset the iPod, press and hold both the Select (Center) and Menu buttons together long enough for the Apple logo to appear.
If still no luck, try a complete restore in iTunes. This process will wipe all content from the iPod, set it back to its original defaults settings, and restore a fresh copy of the iPod's firmware onto the device. This procedure will help rule out a softare glitch or data corruption.
Otherwise, it's likely that something is physically wrong with the clickwheel, meaning you'll need to take it to your local Apple Store/Genius Bar to have them take a look at it and verify that it is indeed a hardware issue.
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