If illustrator crashes or computer crashes when reopening illustrator, the linked files are all missing. A dialogue box says something like "Can't find missing file "." . . " as it would normally do if a linked file was moved out of the original folder. However, rather than the original filename showing between the "speech marks" all you get is a full stop/period "." and all linked files are missing. If you save the file that file then it will always have the items missing and there is now indication of what the file is in the linked files panel as it now says that image is embedded or missing.

This can also happen when exporting for web and you cancel the save as it's saving using the escape key, boring the save. All linked image files then disappear and you have no indication of what they originally were by relinking.