! next to many tunes after I-Tunes upgrade

I updated to the newest I-Tunes version at their request. After doing so, many of my tunes have a ! next to them but the majority of the tunes don't. The funny thing is, these tunes were all in the same library. Arrrggg!!!! Dang it- it seems like every time I upgrade I-Tunes something gets hosed. I definitely have a love-hate relationship with I-Tunes! Can anyone help with this? Many of the tunes I generated from my own studio or are from CDs I own. Some tunes are duplicates and one has the ! mark while the other doesn't! I don't get it.