exclamation symbol

I bought my iPod in October. I have downloaded purchased music onto it as well as some music that I already own. The player has been working just fine until recently. I noticed that a few of my listed songs on my iPod had a symbol in front of them - - - a red circle with an exclamation point inside of it. I can no longer play the songs that have that icon in front of it (it is not the symbol with the folder and the exclamation mark). The songs are a combination of my own CD music and music I have downloaded from iTunes. Any suggestions? ideas? answers?

Thanks!