Usually this is caused by a corrupted file. So if one of those songs you added is the culprit, you'll have to exclude it to get the sync to work again. Try not adding the file, and playing it in iTunes, or another media player to check to see if it is bad.
Other causes are bad cable, bad USB driver, hub between the computer and the iPhone.
Have you backed up your iTunes Library this week? If not, you are risking losing your music and playlists....