Do not Import (pull up?) anything.!!!
Again- Do not try to re-import your photos, until you have searched for Catalog files (?????.LRCAT).
Do you have a Catalog file (your "Library") from the 'old' computer? Or a Backup copy of your catalog file on the external drive?
If you do, then read this link to re-establish your library- https://www.lightroomqueen.com/how-move-lightroom-to-new-computer/
If you do NOT have a Catalog file whatsoever, then you will have to 'start-from-scratch' with a new catalog and hopefully the XMPs will restore some of your editing work to the photos, when you import them to the new catalog.
So, I found a "Catalogue" file from a previous computer (although it may not be all of the files - I had to go through 2 computers to get to my current one - fun!). But I've been editing new photos on my new computer. Lightroom says it has to close the current Catalogue and relaunch to open the old Catalogue. So, I can't just add it? I have to choose New or Old?
If you want to combine catalogs, you must open one catalog and then choose the menu option to import from another catalog. That will enable you to combine images from multiple catalogs.
Thank you. That might work. It looks like I was using an older version of Lightroom on my old computer and it says it can't convert it and import it. So, I'll probably have to install old Lightroom and then upgrade and then import...
Two Catalogs can be merged into one by using the Menu option -[Import from another Catalog]
This would restore all your 'old' editing with the work done on 'new' photos.
You may (will) have problems with files that may have changed location (show as 'missing') but you can fix these problems also.
Before you attempt a merge I suggest you read all four articles "Stage 1,2,3,4" from the link-
Fix 'missing'- https://www.lightroomqueen.com/lightroom-photos-missing-fix/