Do you want to move the actual catalog (the .lrcat database where all the data about your images - but not the images themselves - is stored), or just move the image library?
If it's just the images that you want to move, and assuming the copy that you made on the external drive is still there, right-click on the "Photography" folder in the Folders Panel in the Library module and choose "Update Folder Location". A file browser window opens, use that to navigate to the "Photography" folder on the external drive, and select it. That will update the catalog database to re-link to the images on the external drive.
If that's not what you want, or if you've deleted that copy of the image library on the external drive, let us know.
If you are new to Lightroom Classic, Victoria Bampton has made a free Quick Start Guide (register at lightroomqueen.com to download a copy).