In the instructions we tell people that dnf update is wrong way to do this distribution upgrade. We also show there is more to this than one command. If my guess is correct this was easily avoidable. Now this becomes a wonderful learning experience.
Best guess is that at the end of your dnf transaction you were asked some quesions and the way you answered wiped your /etc/shadow file. That file contains all your passwords and user information.
So your system is recoverable but you will need to set new users and passwords even if they are the same ones you had. Start with internet search “Linux how to recover root password” and find a tutorial on this you like. You will need to recover/set a new root password first and go from there.
You will want to add at least one user as well. You can do internet search for “Linux How to add user from command line”.
This all assumes I have correctly guessed what caused your issue. If I am correct you don’t have any users either and you have to enter the uname in sddm login screen.