Hello and welcome,
Sounds like a partial update to me. From TTY please run this commands as your user:
sudo pacman-mirrors -f && sudo pacman -Syyu plasma-desktop sddm qt5-base
then reboot the system by
systemctl reboot
If the issue still persists, from TTY once again you run:
systemctl stop sddm
Try to start your session with:
startplasma-x11
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