Syncing problems

My Nano has recently stopped syncing correctly. I am using the same computer, but did recently start using a Nike+ attachment with my iPod, and also recently updated iTunes. I'm not sure if one of these things caused my problem, or if it something else. All I know is that I have purchased about 20 songs that I can't get to sync onto the iPod, and the library is no longer recognizing the 400 I already have on there. Help!