Q: I-Tunes Exlcamation Mark dropping music

I am getting an exclamation mark on my I-Tunes music library (in my PC) today on about half the songs I try to play.

It won't let me play them and I haven't deleted any of them.

What's the best way to proceed from here? I am afriad to hook my I-Touch up for fear it will sync and remove the songs that exist on my I-Touch.

Any Help?

Thanks

Jerry