I think your issue is related to what I posted in the 5-18 update thread @ [Stable Update] 2024-05-18 - Linux-Firmware, PHP, Gitlab, Qt6 - #80 by Daniel-I
If what I found is correct, it’s an issue that currently exists under X11 and not Wayland. And as @cscs pointed out, kscreenlocker is involved, and there was mention in the link I provided there that made it sound like they may need to expand it’s logging (or was it powerdevil?) to expose the root cause.
Like you mentioned, although the screen is black/blank, you can type your password in and get back to the desktop session. I also mentioned in my posts that if I switched away from TTY2 (like TTY1 {ctl-alt-F1}, TTY3 {ctl-alt-F3}, etc…) and then switched back to TTY2 (ctl-alt-F2) the blank screen would be replaced by SDDM.
So, we may need to wait for the upstream fix if X11 is preferred (I’ve returned to X11 as plasma6 broke using gamescope for me) over a Wayland session. Use the TTY-toggle if you want to force refresh SDDM to show, or skip that and type in your password like normal; assuming it’s there.
Also, since you mentioned powering down your running PC, I’d like to mention another option that runs way less risk of file corruption that could render your PC unbootable and/or corrupt your data files… REISUB as explained @ [HowTo] reboot / turn off your frozen computer: REISUB/REISUO