After the recent update all nvidia drivers except 455xx and 390xx are dropped due nvidia’s policies. Because they are not compatible with the newest kernel.
If you really need the nvidia 340xx driver, you will have to use an LTS Kernel like 5.4 and package the driver yourself.
Otherwise i recommend to use the nouveau driver if possible.
Also try the 390xx driver. Maybe it supports your gpu.
mhwd -r video-nvidia
mhwd -i video-nvidia-390xx
To over come your actual problem is removing nvidia driver completely and also mhwd and reinstall it.
This quote is in german, but maybe you understand the commands: