Hi,
The same happened to me, the wp-admin disappeared but the slug didn’t work either.
I saw in a previous post that we could edit .htaccess file, so I did, and it works! THANKS A LOT!
1. Through FTP, go to ‘public_html’ folder
2. Download ‘.htaccess’ file
3. Open it in notepad
4. Delete all that block after # END WordPress
5. Access your default Admin page again (hopefuly)
Good luck