Previously when I chose the All Photos catalog in the left panel, there was a dropdown in that panel with years and months to choose from, but the dropdown is no longer showing. How do I get it back?
What is the name of the software are you using? What version NUMBER?
Please be more specific ... was the dropdown the Folders Panel where the folders are named by capture date? Or was it something else?
My Lightroom is up to date 11.4.1. The months and years came up when I clicked on the All Photos catalog
If you are talking about Lightroom Classic, then I believe you are talking about the Library Filter bar. That bar comes up if you click on 'Text', 'Attribute' or 'Metadata' and it disappears when you click on 'None'. Dates are metadata, so click on 'Metadata'. You will see several columns, that you can change yourself by clicking on the column header at the top left of the column. You can also add or remove columns by clicking in the top right.
If this is not what you meant, then perhaps you are confusing Lightroom Classic with Lightroom desktop. Lightroom desktop has the dates in the top left panel:
Just for furture reference, The items shown in the "Catalog" panel are not catalogs as the panel name would imply. They are "special collections" that are created and managed by LrC itself.