This might be too obvious an answer but have you checked your iPod and iTunes settings to make sure nothing has changed? Connect your iPod and click on it's icon in the iTunes source list (the narrow pane on the left). This will bring up the preferences tabs in the main pane on the right. The first tab you'll see is Summary, look for "manually manage songs and videos", if there is a check mark there then the iPod is set to update manually and you have to drag songs from the iTunes library to the iPod to load them on. If you want to sync automatically you need to remove the check mark from there (I would also remove the "enable disk use" check mark unless you want to use the iPod as a removable storage device) and click the Apply button in the right hand corner. Also check the iTunes preferences, go to iTunes>Preferences>Devices and see if the box "Disable automatic syncing for iPhones and iPods" is checked.
If it's not set to manual and automatic syncing is not disabled and still won't sync then try restoring your iPod. Restoring will erase the iPod's hard drive, reload the software and put it back to default settings. Once the restore is complete follow the on screen instructions to name the iPod and automatically sync your songs and videos onto the fresh installation. Press Done and the iPod will appear in iTunes and start to sync. If you want to update manually or using selected playlists uncheck the box beside the sync automatically instruction and press Done, it will default to manual mode and you can choose whatever setting you like:
Restoring iPod to factory settings with iTunes
