If your installation was originally in D:\MSFS2020\Official but that folder is now empty, it must have been deleted for some unknown reason. Did you press the ‘Verify Game Files’ option in the Steam MSFS properties? Or maybe something somewhere got corrupted?
Either way if that folder is now empty, I would recommend verifying the MSFS install via the Steam option (this would normally delete your game installation, but at this point you have nothing to lose), after doing that the only thing you can do is reinstall the simulator from scratch into that directory. Sorry there isn’t a quick fix 