Collection Viewer is blank

[barneagle]

I hope this is a very basic question. I've just switched catalogs and now have a completely blank Collection Viewer. Whatever I select in the Catalog Area (on the left) the Collection Viewer is still blank. For example, if I select "see all images" under Timeline or if I select any Category the Collection Viewer is still blank.

I've tried switching back to my main catalog (which was working perfectly a few minutes ago) but the Collection Viewer is still blank.

What can I have done wrong?

[barneagle]

For example I've just selected "see all 628 images" under Timeline on the left. The Collection Viewer has a single tab at the top reading "States (All)" and the main bar at the top of the Collection Viewer has the has the usual icons for Filter and so on and then reads "states (All)" with "628" underneath but the main area below is blank.
The zoom level is 100

[Hert]

What happens after you restart the application?

[barneagle]

Thanks, Hert. That's fixed it.