my photos app says it CANNOT FIND THE LIBRARY
My photos app is saying the library cannot be found
MacBook Pro 13″, macOS 10.15
Is the library still where you last left it? By default, it would be in the Pictures folder in your userid's home folder (which is usually on your internal drive).
If you quit Photos and relaunch it with the "option" key held down, you should get a list of visible libraries that Photos can use. Can you see your library there?