Two possible answers i found....
Are the playlists still on your computer? (This is the obvious question). If the songs have been moved on your computer or off of your computer, they'd need to be put back on and into iTunes.
If your songs are stored locally but the iPod is still not syncing, check to see if you have 'manually manage your own music' checked. If you do, it won't sync automatically, so make sure it's unchecked.
If that doesn't work, try turning on the 'Enable Disk Use' option in the preferences. If syncing still doesn't work, try ejecting (from within iTunes, *not* the Windows System Tray) and connecting again. If that still doesn't work, try enabling manual updating of songs, creating a playlist, and dragging it onto the ipod icon in iTunes.
I hope that helps. Let me know if it doesn't.
Other answer i got was
1. On the left menu bar in iTunes, go to your iPod (the thing will be called whatever you named your iPod)
2. Near the top of the screen, there will be tabs. Click on the second one that says 'Music'.
3. Now, make sure 'Sync Music' is checked and directly below it, make sure that 'All Songs and Playlists' is checked and NOT 'Selected Playlists'. Your iPod, if connected, should start downloading immediately.