There are a number of reasons why iTunes might not recognize your iPod.
ITunes may not recognize your iPod if it is not connected properly. There may be a problem with the cable or the USB port, or they may simply not be plugged in all the way. If you are connecting your iPod through an external device, such as a keyboard, rather than directly to the USB port, check that the device is fully charged.
If your iPod is low on power, your computer may not recognize it. Always let your iPod charge at least partially before trying to connect it to your computer.
IPods formatted for Windows cannot connect to a Mac and vice versa. In addition, your computer must meet the system requirements to use iPod.
To find out if the problem is with the iPod, the wire, your computer or some other factor, try connecting your iPod with different cables or to different computers.
Sometimes all it takes to fix a connection problem is restarting your computer. If this doesn't help, try reinstalling iTunes. As a last resort, restore your iPod to its original settings. (This will delete all of the data stored on it, so remember to make frequent backups.)