But what was the actual location of the files (not the aliases or shortcuts that populate the colection)?
If they were on a CD or DVD, for example, the collection won't find them. The volume where they reside must be mounted.
Another example would be if the original files are on a FireWire or USB external drive that is no longer mounted, or on a server that has been disconnected.
What platform are you on? Macintosh or Windows. More details are needed.
Please read this FAQ for advice on how to ask your questions correctly for quicker and better answers:
In CS5 there was a bug that if the path/filename contains an apostrophe, all files after that will not be shown.
Make sure you have all the updates installed.
I did write a script to try and fix these errors here..