When you open Windows Movie Maker, source files are sometimes missing in the Collections pane. Windows Movie Maker collections and projects do not really contain the media files that you work with; rather, projects and collections only reference the digital media files wherever they are located, such as on a CD, hard disk, removable media, or shared network location.
Some of the reasons why clips might be missing in Windows Movie Maker include the following:
The CD, hard disk, removable media, or network location that Windows Movie Maker uses to reference the file was removed or is no longer available.
The file was renamed.
The file was moved to another drive or folder.
The file was deleted.
In the Contents pane, click the file that is missing or could not be found (that is, the file with a red X).
Click Edit, and then click Browse for Missing File. If Windows Movie Maker finds the file, the red X disappears.
If Windows Movie Maker cannot find the file, click File, click Import Media Items, and then browse for the missing file.
You can also use Windows Explorer or Windows Search to try to find the file.