Downgrading kernel problem - Unable to boot after (Terminate Plymouth message...)

[Aravisian]

@doalex

Learn more about Ubuntu Pro at https://ubuntu.com/pro

I do not see any repositories listed that are Proposed Repos; but I do note that you are being advertised to use Ubuntu Pro...

It looks like the Nvidia 340 driver is unsupported on later kernels. You may need to use a kernel that still supports it.
Which explains:

@doalex

I tried to downgrade using Synaptic, choosing kernel 5.4

You also said that you can boot 5.4...

@doalex

I also see with uname -r that I'm on kernel 5.4.

So you may need to Lock In the 5.4 kernel to make sure that you stay on that kernel and always boot into it. Using the Recovery Menu Prompt:

[HOW TO] set an older kernel to default boot

You may need to use an earlier kernel than the current kernel used by Zorin. This method allows you to set the kernel that you will boot into by default. To set the new kernel to default: Boot into the kernel you wish to be Saved. Open terminal and run: sudo nano /etc/default/grub Change GRUB_DEFAULT=0 to GRUB_DEFAULT=saved Hit enter to drop down a line. Add GRUB_SAVEDEFAULT=true ctrl+x to exit, then the y key to say yes to save, then the enter key to save as current configuration. Ru…

Remove the 5.15 kernel:

sudo apt remove linux-headers-5.15.0-84-generic linux-modules-5.15.0-84-generic linux-modules-extra-5.15.0-84-generic linux-image-5.15.0-84-generic

Reinstall the Nvidia 340 driver:

sudo apt install nvidia-340

You do not need to use DKMS since you are locking in the kernel and not allowing it to upgrade.