Hello,
Use this command from terminal to update everything and reinstall (in case something got broken) KDE Plasma related packages:
sudo pacman-mirrors -f && sudo pacman -Syyu plasma-desktop plasma-framework kf5 qt5-base
Reboot after that, add a new panel if is missing, check Desktop Session to start with an Empty Session, reboot again and see if the Panel is loading, then you can change back the session if you prefer to load the saved session …