I had the same issue because the recent ICU update failed when I tried to install it together with munspell or something like that. There was an error during the installation and it broke everything, even pacman, so I couldn’t upgrade or install anything or do whatever with pacman. I couldn’t even open Nautilus and other software. All depend on the libicui18n.so.58 library.
I had to manually download ICU (or go to /var/cache/pacman/pkg) from the website, extract the files and rsync all of them into the /usr directory. I also did the same with several other software I updates before this issue. Luckily that solved everything and then I did a full update of all the available updates and everything was fine.
A few examples of the commands I had to use:
tar -xvf theicufile.tar.xv
rsync -a bin /usr/bin
rsync -a lib /usr/lib
I did all the rscyncing per folder to avoid any extra issues, because rsync was still quite new to me.
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