Morning walfredo,
Copy and paste the blue text into the search bar: [=ismissing([filename])]=1
That answers your question, but please read on...
Had you used media center to do your re-organising, your library would have remained intact, with fully connected files, along with their original library information which includes such stuff as date imported for all your files, number of plays for your files, date last played for all your files etc. etc.
One of Media Center's most powerful features is its file management tools. Right click on a file or file selection and look at the options presented to you under "Library tools". The "Rename, move & copy files" tool is incredibly powerful and allows you to move files around your drives in almost any manner you could think of, so long as your tags are right.
Have a look at the "My Computer" branch of the Media Center tree. It's not my preferred choice, but you can drag and drop stuff around in there, and MC will move the files on your discs, while at the same time, maintain all of your library links to the moved files.
Another option is to directly edit the "Filename" or "Filename (Path)" field from either the tag window within MC, or directly from the file list itself (slow-double-click-on-the-field-in-the-list).
Once you familiarise yourself with the choices open to you, trust me, you will never again use Explorer to organise your media files.
Do some searches both here and in the MC wiki for more help and ideas, or just start a new thread if you're having trouble finding the answers you need.
Your library will become quite important to you, and you'll be pleased to hear that MC makes automatic library backups for you. My library is getting on for eight years old now, and it gets backed up every night on a rolling seven day increment. Thankfully, I've never needed to fall back on a backup set, but you cannot be too careful!!
The only other thing I'm wondering, is why the auto-import did not remove the missing entries from your library... are your files living on a networked drive?
-marko.