From the Answers post/solution:
Note: Resetting office will delete your local phone documents, so please e-mail them to yourself to keep a copy. It will also remove any pinned items from your start screen.
- Go to Settings
- Go to the Applications pivot
- Select Office
- Press the Reset Office button (if you get a message that Office could not be reset, reboot your phone and try again).
After following the above steps, files and notebooks should open properly from SkyDrive. If you had any documents pinned to the start screen, they will be removed during the reset process and need to be repinned. We apologize for any inconvenience.
This sort of snafu is disappointing, to say the least - seeing cloud-stored documents appear to 'disappear' rather dents everyone's confidence in the entire concept - even if, in this case, it's more of an optical illusion. And yes, play attention to the bit in bold, above, too!
(Via the Windows Team Blog)