One of the features that's changed in Lightroom 3.3 and 3.2 from 3.0 is how the Library Filter works. In version 3, the setting was not "sticky." That is, for each search, you started fresh. In 3.2, however, the filter returned to how it worked in Lightroom 2, where it was sticky.
So now, Lightroom's main window shows the results of a previous search until you enter new search criteria.
If you were used to how this worked in Lightroom 2, you may be happy to hear of its return. If you never used that version and joined the party with the release of Lightroom 3.0, you may be confused about what's going on—and panic when your photos seem to disappear.
Adobe's Tom Hogarty explains the double-switch—and the thinking behind it. Btw, this has been a subject of pretty intense debate among beta testers. If there was a simple, intuitive fix, Adobe would have implemented it long ago. But there's not. Learn why.