Grub: Boot frozen

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The boot process with grub may be frozen at different stages, for a variety of reasons. This page aims to list possible scenarios, and redirect the user to a proper fix, according to scenario.

Frozen at "GRUB" after nvidia driver upgrade

Symptom:
boot computer, but before any grub boot menu is displayed, the monitor is black with the only letters "GRUB" written on it.
Context:
This happened just after a broken nvidia driver upgrade.

The general steps to fix the problem were:
1- boot into the system using a rescue disk.
2- In a console, create a script with the following commands and run the script, or manually enter and run each command individually:
Adapt the mount points and device name to suit your environment:

#/bin/bash

mkdir /mnt/gentoo
mount /dev/nvme0n1p2 /mnt/gentoo
mount -v -t proc proc /mnt/gentoo/proc
mount -v --rbind /sys /mnt/gentoo/sys
mount -v --make-rslave /mnt/gentoo/sys
mount -v --rbind /dev /mnt/gentoo/dev
mount -v --make-rslave /dev /mnt/gentoo/dev
mount -v -o bind /tmp /mnt/gentoo/tmp
mount -v -o bind /dev/pts /mnt/gentoo/dev/pts

chroot /mnt/gentoo /bin/bash

3- depending on your system, perform the upgrade necessary, in order to rebuild the video drivers.
emerge --update --keep-going --deep --backtrack=3 --changed-use --tree --ask --verbose-conflicts @world
In the present case, the x11-drivers/nvidia-drivers indeed needed to be rebuilt. After rebuilding it, exit the chroot environment and then reboot the system.