The ! Means that the link is broken between what is shown in the Library and where the file is located. You will need to relink the items. You can search you HD to see where the files are located then use CMD-O to relink.
Did you move any items using the Finder?
iTunes is a data base manager and if you move items outside of the iTunes interface then iTunes might not keep track/find the move.
You might find a script here to relink the tracks with ! - http://dougscripts.com/itunes/
As far as I know, nothing has been moved using the Finder.
It started out with just a few songs having the ! but now, more and more are showing up with it.
I have an external hard drive, which has never been disconnected, but now, it doesn't show up on the Finder.
I'm not sure I know how to do what you are recommending; can you give me specific steps to try?
Thanks so much for offering to help!