Windows Media Player not syncing music to Sansa Clip+?

I have a Sansa Clip+, WMP 11, and a Vista Home Premium package. For at least 6 months, I have been able to sync music to my MP3 with no trouble. My dad owns a Clip+ as well, and last night, all of a sudden, it stopped syncing to his MP3. I checked with mine to see if the problem was with just his MP3 player, and it wouldn't sync to mine, either. When I drag songs over to the sync list (it DOES still recognize my MP3) and click 'Start Sync', it will do nothing and immediately say it's finished syncing even though it hasn't done anything, whether I try one song or a full list of songs, it doesn't work. WMP functions normally other than this, it still burns and rips CDs fine, and my MP3 player (and my dad's) function perfectly fine. What could have caused this, and how can I fix it? My dad is very impatient and will go ballistic if I don't find out how to fix this soon. Thanks! :)