Pamac is stuck at ‘preparing’ stage regardless if I want to install a program or to update the system. It’s been behaving this way for a couple of months and I used to update in cli with pacman -Syu which works flawlessly.
It’s not frozen, it’s responsive, it only shows “preparing” and can’t be closed.
Try refreshing your mirrors… 
sudo pacman-mirrors -f 5 && sudo pacman -Syyu
Let the above commands take their time and finish cleanly. After that you can try using pamac to add/install applications.
The mirror list has been prepared and saved in /etc/pacman.d/mirrorlist.
Syncing database
core 100 %
extra 100 %
community 100 %
multilib 100 %
Starting full system update.
Nothing to do.
It didn’t work and pamac is still (again) stuck.
Try to update pamac bases : icon on right (3lines) > update database
Doesn’t work either. Still stuck.