I experienced the problem with Neon rebooting to a black screen upon successful completion of the upgrade to 24.04. GRUB was inaccessible, as was any form of TTY (due to their being no display whatsoever).
I resolved the issue by SSHing into the affected PC and performing the following:
sudo add-apt-repository ppa:graphics-drivers/ppa
sudo apt install nvidia-drivers-555
Once the download and install of the nessecary drivers had completed, I performed a sudo reboot and all was good.