Probably other gtk3 packages were also installed alongside thunar-gtk3 and other conflicting gtk2 packages removed, and since gtk3 is not really stable yet, this is what could have caused your issue.
Follow this guide to login to tty terminal, then remove thunar-gtk3 and install thunar, this will probably reinstall the previously removed gtk2 packages as well. If not, check in /var/log/pacman.log which packages (if any) were removed when you installed thunar-gtk3 and reinstall them manually.
To install thunar-gtk2 from the tty, which will also remove thunar-gtk3
sudo pacman -S thunar
then to reboot