Locating Missing Files And Folders Within Adobe Lightroom

Question - My thumbnails have a ? symbol on the top right corner – what does this mean and how do I remove it?

Short Answer -
The ? symbol is used to identify a file that Adobe Lightroom can no longer access; probably because it has been moved using one of the Operating System tools such as drag/drop or possibly the Move command. Unfortunately, Adobe Lightroom currently has no way of tracking such changes, so the ? symbol is displayed to warn the user that the file or folder containing the file can no longer be accessed.

Long Answer - Locating Missing Files and Folders
Even though Lightroom now uses the Folder paradigm rather than Shoots (as was the case during the public beta), it doesn't display the full disk/folder hierarchy in the tree form that most users would be familiar with. This means that moving folders from disk to disk or even to a section of the hard disk not shown on the Lightroom Folders panel very often needs to be done using OS level tools such as Drag/Drop or even the Move command. The problem with this approach is that Lightroom can easily lose track of your files, and this results in the ? symbol being displayed.

Computer-Darkroom.com provides a detailed solution to this issue with Adobe Photoshop Lightroom.