You are using Apple TV software version 3.0 and all of your movies, TV shows, and songs appear to be missing, or You are using Apple TV software version 3.0 and all of your movies, TV shows, and songs appear to be present, but you have not yet updated to Apple TV software version 3.0.1.  

Follow this link:
Apple TV 3.0: Update to Apple TV 3.0.1 or later to prevent content from temporarily disappearing until re-synced

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