I have an article for you here that can troubleshoot that issue for you. While the article is short, it will check one important, often overlooked step on the iPod shuffle:
If that is unrelated to the issue you are having or does not affect the issue, there are some relevant links at the bottom of that article that I will lay out here:
Mac OS X users:
- iPod not recognized in iTunes and Mac desktop
- What to do if your computer won't recognize a FireWire or USB device
Thanks for being a part of the Apple Support Communities!