Over the years I have kept my photos on my computer hard drive. My practice has been to edit the photos from a shoot and create a Collection of those edited photos. I have decided to follow the practice of keeping photos on an external hard drive. I moved all of my photos from 2017 to that external drive. My Collections for Ireland disappeared from Lightroom. (Collection title remains; no photos, books or slideshows in the Collection.) Re-importing the 2017 photos from the external drive into Lightroom does not re-populate those Collections. How do I re-populate those Collections with all of the edited photos, photobooks and slideshows?
One way, which you may not like, is to manually re-populate the collection by dragging and dropping within Lightroom Classic.
The other way is to use a recent backup of your catalog file where the Ireland population is populated. Open this backup catalog. Then select the collection by clicking on it, then File->Export as Catalog, and save this resulting new catalog. Open up your original catalog (not the backup) and then File->Import from Another Catalog and select this new catalog.
ADVICE: Never re-import photos. Re-importing never solves problems, and usually creates more problems.
I moved all of my photos from 2017 to that external drive. My Collections for Ireland disappeared from Lightroom. (Collection title remains; no photos, books or slideshows in the Collection.)
It sounds like you moved the photos on the desktop, outside of Lightroom Classic. The next time Lightroom Classic looked for them, they were not in the folder where they were cataloged. One good way to resolve this is, in the original catalog, right-click their original folder and choose Update Folder Location. That would show Lightroom Classic where they went. More info:
That re-associates the catalog with the images, and also re-associates all of the metadata connected to the images including their collection membership. Because the problem is that some catalog-level info like collection membership is stored only in the catalog, not with individual images. Re-importing never helps because it’s like starting over: No previous history or metadata is assumed to belong to newly imported images even if those filenames already existed in the catalog (especially because they are in a different folder), and the collection info was not in the images anyway. Update Folder Location is what reconnects images with their metadata.
If the new parent folder on the external volume is already listed in the Folders panel in Lightroom Classic, then moving across volumes is even easier: Just do the move within the Folders panel, not out on the desktop. When the move is done within Lightroom Classic, no info is lost (collection membership is carried with the images, etc.).
In short, any time you move images “behind the back” of Lightroom Classic it will lose track of them, so move them within the Folders panel if possible and if not, move a folder on the desktop but then immediately use Update Folder Location in Lightroom Classic.