Somewhat similar thing happened in my situation. I upgraded from Mojave to Catalina and Photos 'worked' though I lost stretches of time randomly, like August - December 2017. No rhyme or reason.
Depending on what you upgraded from, you've probably already migrated from iPhoto to Photos, because iPhoto got pushed aside awhile ago. You're probably just calling it 'iPhoto'...which I do too.
So, I tried repairing my library (which did not do anything) and I tried opening a different library at startup (no dice either). The photos weren't in the trash or any other hidden folder...just gone. Thanks Apple!
Luckily I had some backed up on Google photos, but they are at a lower resolution, so not the 'fix'.
My Microsoft Office disappeared as well from the startup menu, but it was under 'Applications' so perhaps it's still there?