We dropped 340xx driver of Nvidia, as it is not supported by Nvidia anymore.
However you can get the sources from us and compile the needed drivers on your end:
sudo pacman -S git wget base-devel
git clone https://gitlab.manjaro.org/packages/extra/nvidia-340xx-utils.git
cd nvidia-340xx-utils && makepkg -si && cd ..
git clone https://gitlab.manjaro.org/packages/multilib/lib32-nvidia-340xx-utils.git
cd lib32-nvidia-340xx-utils && makepkg -si && cd ..
git clone https://gitlab.manjaro.org/packages/extra/linux419-extramodules/nvidia-340xx.git
cd nvidia-340xx
You may want to higher the PKGREL in the PKGBUILD as needed
makepkg -fsi
… then you should have the working driver again. If you have issues, post about them. nvidia-340xx needs to been rebuilt with every kernel update.