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If an exclamation mark next to iTunes media in your library means that either the media is missing from the computer or iTunes is not directed to where the media is located. Here is an article that will help you locate the missing media. I have also included a list of icons in iTunes 11 as it will direct you to the best course of action.
If the media that is missing has been purchased from the iTunes Store, you can delete the problematic media and download it again free by following the steps below.