Photo Library Issue OS Catalina

I have an iPhoto file on an external drive with old photos that were not part of my current system Photo library. I wanted to pull these photos and put them into folders by year, so I opened the file, and I did just that. Fine and dandy. When I went to open the current Photos file on my system hard drive -- I got a prompt (that I honestly don't remember what it said) and I clicked OK. Now, all of my newer/current photos are gone.

I thought I had done something similar before without issue (i.e., opened an old "archive" photo file and then returned to current photos), but this time, the only file I can find contains a small number of photos from two years ago?? Did I delete or overwrite my current photos somehow? Is this a Catalina thing? I dragged my feet forever, and only recently upgraded to Catalina OS a few weeks ago (was using High Sierra).

Note: I do have a time machine back up from a year ago, on my external drive. I thought maybe I could at least pull that and access some of the in between photos that are gone, but the "restore" option is greyed out.

Any help would be appreciated.