I recently needed to re-export an project i had finished, so returned to the project.
After connecting the drives with the media I relinked the files and most of the files relinked without an problem, but some clips stayed black.
I restarted my computer and now also the thumbnails and in some cases even the filenames are empty in the timeline.
It it pretty sporadic in that they are from different camera's, and divided over different drives.
Ive tried trashing the preferences, moving the project to a different location, exporting an xml and opening that... nothing seems to work..
Sometimes this is indicative of a clip that was referencing a camera original. Go through and figure out precisely which one's are missing. Confirm they actually exist in the Finder and then reimport. If they still won't connect you may have to over-cut with the new imports.