I successfully broke my computer again. Oops. I can watch systemd boot my system and then once my display manager (in my case, sddm) takes over, kaput.
Once that's happened, I can't do anything else. I can't switch to a text console because my displays switch themselves off, nor can I start a graphical session because, again, my displays switch themselves off.
Rebooting yields this:
(and clicking the reboot button does nothing)
I managed to fix this by booting into EFI shell, then specifying runlevel 3 by booting the kernel's EFISTUB. My EFI shell didn't like the leading slash for some reason. Whatever.
Once that was done, a pacman -Syu (which updated the nvidia package, among some other things)
and a mkinitcpio -P linux got me back up and running.
Simples!