are you compiling from source, or using the EGS install?
what was the previous engine version you were using?
did your project start as Blueprint then converted into C++ or start as a C++ project?
are you using Source Control? (do you login to source Control before opening the editor?)
are you including “Starter Content”? (at some point in engine versions the “import starter content on first startup” became every editor startup)
are you able to create a new project through the editor, and open that one? (sometimes you need to get all the way to the editor on some project for everything to “just work”)