lost? photos

Your Catalog, its records of which photos are imported, the work you have done on those photos are all still fine. Just, the source files now cannot be found by the Catalog at their previously recorded locations. So LrC needs to be informed about what the new locations are.

Best case, all your photos were previously stored in some sort of a folder hierarchy under a single containing parent folder - "photos", for example. And a corresponding parent folder now exists on the external drive, with the identical arrangement of folders and photos within. It does not matter if the name of this single common parent has changed - but ideally the rest will not have changed. If this common parent folder that formerly existed is not showing in the Catalog's Folders panel, you can go to a top level folder that is showing, and do "Show Parent Folder" until you do see that. Then do "Find Missing Folder" onto that, and browse to the corresponding folder that is now on the external drive.

This will update LrC's information about all the photos and folders that it is able to match up with source files once more. If the folder arrangement has changed then you can either try to first organise everything back to correspond again to the folder structure that currently shows inside the Catalog - or else, just re-browse all the various parts of your folder arrangement bit by bit.

• Readdressing (as above) is the best and quickest way, and loses nothing.
• Removing the photos and then re-importing them at the new location will work but is definitely not great, since this will lose all, or at least some, of the work you have done onto these photos.
• To leave the "missing" photos in place and then ALSO import the files living in the external drive - that is actively a bad idea and will require a tedious cleanup just to get back to where you are now.

You have to tell LrC where you moved the photos to. See this article: https://www.lightroomqueen.com/lightroom-photos-missing-fix/

Note: this and all the other procedures you will find online will only work if you maintained the exact original folder structure when you moved the images to the new drive. If you reorganised at the same time you'll need to tell us the extent of the reorganisation.

Do you have moved the photos from the hard drive to the external drive within Lightroom or outside?

I think the thumbnails in the library shows an exclamation mark. If yes, then take a look here:

How to find missing photos in Photoshop Lightroom Classic (adobe.com)

Lightroom thinks my photos are missing—how do I fix it? | The Lightroom Queen