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This is usually indicative that the absolute Path to the media has been broken. In the PREL, one has links to Imported media, which are absolute Paths.
This article for more detail on PREL files: http://forums.adobe.com/message/3397716#3397716
Next, this article addresses "Media Offline:" http://forums.adobe.com/message/3561281#3561281
Finally, for "Where is file ____ ?" see this article: http://forums.adobe.com/message/4598705#4598705