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There are several programs, like Undelete Plus, available to recover deleted files. It may be worth the investment if the files are important. Try out the free versions first to make sure the program works. These programs also work if the recording media has been accidentally reformatted.
Corrupted files are another story. This can happen if the cameras memory looses its index. The thumbnail files are stored separately and therefore, still appear on the camera. Unfortunately, there is no image or video file associated with it ad the cameras memory doesn't know where to go, therefore, the question mark. As I mentioned earlier, a good undelete program may be worth the investment. Some of these will rebuild the index file and, if the space the video file hasn't been recorded over, will reconnect the file to the new index.