Robert, I do not know if it is your video or kernel (4.20) or upgrade or grub that is the issue.
I can only propose some measures to try to work these out and I am not sure if they can work out for you.
If you have 4.19 (I am not sure if you now have and you said 4.19 works for you previously), use 4.19 otherwise you have no choice but to use 4.20 to boot into prompt using the fallback initramfs.img
Please refer to this first post here and do not use the [Simple First Start] as this will not work for you. And in step 6, use "initramfs-4.19-x86_64-fallback.img" and add a "3" (no apostrophes) at that linux line to boot to prompt.
Also do the commands in step 10 which requires a clearing of grubenv and recreation of same. Don't forget to 'grub-install' and the 2 additional uefi commands (if uefi).
Oh... make sure you updated your system first (pacman-mirrors, pacman -Syyuu) when/if you booted up to prompt.
If you can boot up to prompt, there's hope and your grub will be fixed as well. If you cannot boot up normally subsequent to this, it is more than just your grub. Try fixing video drivers or the kernel with 'mkinitcpio -P' or with installing older kernel.
Whatever, good luck. Cheers.