where my files go in upgraded version of visual studio?

Hello:

I have downloaded simultaneously two visual studios vs2017 and vs 2019 community edition, but recently, many of my project files, such as solution file, project template and config files and source files get lost, leaving only mdb, dep file and assembly file(for dotnet core library project), why so many of my files disapppear and are they related to the upgraded version of visual studio and version imcompatability, and how they are related and how to correct them

thanks