Files disappeared

iPhoto was deprecated by Apple in 2015 in favour of the Photos app.

iPhotos will not work in Catalina as it is a 32bit app, 32bit apps do not work in Catalina.

You are meant to do research before blindly upgrading your mac operating system.

Open your Photos app (Dock and/ or Applications folder) while pressing and holding down

the option /alt key a Choose Library dialogue will open select your iPhoto library and press

the Choose Library button. Your iPhotos images will now be imported into the Photos app.

How did you upgrade to Catalina, installing a new OS over an older OS

should not result in a loss of data.

Could you explain in more detail what you did.