3. add it to the library using the desktop/explorer/libraries interface
4. re-index and wait for it to complete
Number 4) helped for me exactly on same problem:
I just removed My Pictures folder from Pictures library and put it back; I'm guessing that re-indexing did the trick.
Although a little bit worried why this happened in first place. I did absolutely nothing: It was working 2(*) days, then no new programs installed, not even new photos imported, maybe some Windows updates, and kaboom. Everything vanished from Photos application
(Everything from my computer, SkyDrive was working all the time.)
(*) My Windows 8 installation is now 5 days old. It took 2,5 days to find this answer. Uninstall+Reinstall and other stuff proposed didn't help.
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