This happens if the file is no longer where iTunes expects to find it. Possible causes are that you or some third party tool has moved, renamed or deleted the file, or that the drive it lives on has had a change of drive letter. It is also possible that iTunes has changed from expecting the files to be in the pre-iTunes 9 layout to post-iTunes 9 layout,or vice-versa, and so is looking in slightly the wrong place.
Select a track with an exclamation mark, use Ctrl-I to get info, then cancel when asked to try to locate the track. Look on the summary tab for the location that iTunes thinks the file should be. Now take a look around your hard drive(s). Hopefully you can locate the track in question. If a section of your library has simply been moved, or a drive letter has changed, it should be possible to reverse the actions.
Alternatively, as long as you can find a location holding the missing files, then you should be able to use my FindTracks script to reconnect them to iTunes .
Assuming you are in a region where you are allowed to redownload your past music purchases, delete the entry for the missing track from your iTunes library, close and then reopen iTunes, go to the iTunes Store home page, click the Purchased link from the Quick Links section in the right-hand column, then select Music and Not on this computer. You should find a download link for your track there.
While downloading select Downloads in the left-hand column and make sure Allow Simultaneous Downloads is unchecked.
If the problem persists, or that facility is not yet available in your region, contact the iTunes Store support staff through the "Report a problem" links in your account history, or via Contact Support.