Hi!
If you just did an upgrade then your photos shouldn’t be gone so no worries there.
If you try searching for “iPhoto” in finder and click “This mac” so finder searchers the entire mac does the library file show up? (The library file have will have the same icon as before)
You could also close your photos app and hold the option key while opening it again and you will have the option to switch libraries. Does the library show up there? (If yes then you can click it)
Here is an article about switching libraries:
https://support.apple.com/en-us/HT204414
Hope it helps!
/Joar