Sounds like your main catalog is corrupted. Use File...Catalog to open the Catalog Manager. Highlight your catalog and hit the "Repair" button, then repeat, using the "Optimize" button.
Yes Catalog Repair and Optimize is one way you can check this. You could give a try to one more thing.
From the Display dropdown present on the top right corner, select Folder Location view and then in the folder hierarchy that opens you can check if the corresponding folders are being shown with a blue card icon (which implies that media from these has been imported already) or not.