A user over at the Adobe Lightroom Forums was having a problem recently. It seems that he had imported some images into Lightroom and now could not locate any of the original files. While he could work on them in the Library module he couldn't do anything else without those source files. The previews were there. He could see the images. But that was it.
We discussed it a bit and tried some tests but nothing. A thorough search of his drives yielded no results. The originals were gone! Then Don Ricklin made a brilliant suggestion that I want to pass on to you. Marc Rochkind is a photographer and software developer. He has made available a free program called LRViewer. LRViewer let's you view the contents of a Lightroom catalog and see the previews and metadata for the images.
One of the features in the program is the ability to export JPEG files created from the Lightroom previews! Just select some of the previews and then choose Export JPEGs... from the File menu.
Brilliant! Thanks to Marc for making such a useful utility and to Don for coming to the user's rescue! LRViewer is available for both Mac and Windows. Go to Marc's ImageIngester site to find out more. While you're there take a look at the other fine utilities Marc has.
It isn't as good as having your original RAW or JPEG files but it will get you a source image as good as the preview which you can import into Lightroom and work on. The moral of the story is twofold: (1) keep track of your source files and (2) BACKUP BACKUP BACKUP!