Seems from the picture that your are from a Mac.
1. Enable Disk use in iTunes.
Basically, just press don't sync on that dialogue, and go into iTunes.
Then, find your iPod and press "Enable Disk Use".
2. Download Senuti.
http://www.fadingred.com/senuti/
Choose the trial version because you will only be using it once.
3. Start Senuti, and it should find your iPod.
4. Select all songs with
+A, and click the transfer button.
5. There should now be a progress bar telling you what it's currently transfering.
6. After it is done, check it really is done by quitting and going into iTunes to check if all the songs are there.
7. Then, click Erase and Sync, and your iPod should now be syncing properly.