How about the images within the folder -- do they show the ? in the grid too?
If not, I had that problem recently, and found that verifying the catalog fixed it. To do that, shut down LR and then hold down the Opt/Alt key while opening it up again. You'll be presented with a dialog where you can choose to test the integrity of the catalog.
The other thing to try is a Relaunch and Optimize from the Catalog Settings dialog.
These steps are easy and harmless to try, even if your problem turns out to be different than the one I had. Let us know if they make a difference.