Take a look here:
Add the kelebek333 ppa or install kernel 5.15. The 390 nvidia drivers from the repo don't work with kernel 6.8.
To purge installed nvidia drivers please use this command:
sudo apt remove --purge '^nvidia-.*'
To add the ppa:
sudo add-apt-repository ppa:kelebek333/nvidia-legacy
sudo apt update
Then install the 390 nvidia driver.
Also make sure that secure boot and fast boot in your BIOS are disabled, and also fast startup in Windows if you have a Dualboot with Windows.