Do you have teh latest iTunes installed? The new operating system will only work with the latest iTunes. So I'd try updating that. You may need to uninstall/reinstall iTunes. You shouldn't lose any content. I've occasionally had to remove/readd iTunes (I have a PC so it's a bit different but same premise)
Once you've updated iTunes, try plugging your device back in, and you may need to try different ports to get your Mac to 'see' your iPod.
You can also give your device a reset. Hold down the sleep and home keys, past when you see the red power down slider, and until you see the silver apple. Let it reboot. That often clears any glitches preventing the computer from seeing the device.