Not sure about this, but maybe this’ll at least give you some ideas…
If you haven’t already backed up, see if you can access your files via FTP (filezilla is free and works great for that) or the file manager your hosting company probably provides.
Back them up on your local computer (and back up to a couple external drives.) You can probably back them up on the server too. Just make a folder and put everything there.
See if there’s a way to redo the upgrade.
See if your host has a way to redo the upgrade.
Check your index.php file. (You can see all your files with an FTP app or your host’s file manager tool.) Look at the code in index.php itself.
In your WP admin area, look at the permalink for the default home page. See if it’s right and edit it if it isn’t.
Not sure any of this will help, but nobody replied to you and I just thought I’d tell you what I’d be doing (in addition to asking here).