That’s a first for me. Too odd to believe it 
.
If you want to fix it, or investigate, you have to do some work.
For example:
- Inspect pacman log (the part from the last upgrade till the end) for error messages or unexpected end of the process.
less /var/log/pacman.log
Hit key End
Type /pacman and hit key Enter
Hit key N (capital N, with Shift+n), until you find a line which runs pacman -Syu, or equivalent (from a helper utility like yay).
You may copy the lines from there to the end of the log file, and post it (here, or in a paste-bin service). Or post the complete file, if you don’t mind.
- Get journal messages
journalctl -b -u sddm
journalctl -b -p3
-
Get sddm log file
For SDDM in X11:$HOME/.local/share/sddm/xorg-session.log
For SDDM in Wayland:$HOME/.local/share/sddm/wayland-session.log -
Get current sddm configuration (other than system/sddm default)
for Xconf in /etc/sddm.conf.d/*.conf /etc/sddm.conf ; do echo -e "\n### $Xconf" ; sudo cat "$Xconf"; done
- Post more details of your system
inxi -Faz