Help! Opened an older iPhoto library and now my current one is missing
Searching for a couple of old photos on a peripheral storage device, I accidentally opened an older saved version of my iPhoto library. When I tried to open the "regular" one on my Mac Mini today, it said a different version was the last used, and to plug in my storage device to get to it. Choosing a different list of photos in the menu, it's like it's a brand new (meaning empty!) library, and I see no choice for wherever my former library is. On a 1-10 distress scale this about a 9--the saved version on the storage device has photos up to 2018, but have I lost everything since then? If not, where is it and how do I get to it? Not saved on iCloud, so that's not an option.
Mac mini 2018 or later