Grub recovery

If, for some reason, your Linux system won't boot, you can spin up a rescue system.
This issue might occur after a kernel upgrade or downgrade.

First of all, if the system uses RAID, you need to assemble it:

mdadm --assemble --scan

Next, mount the appropriate disks:

# lsblk to identify them
mount /dev/md127 /mnt/
mount /dev/md126 /mnt/boot/
# Last md device will be swap, skip that
mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys

Now, for the GRUB commands to work, change the root environment and make it think /mnt is the root. This can be achieved with chroot:

chroot /mnt
grub-install
# If grub install fails use: on all devices
grub-install /dev/nvme0n1 && grub-install /dev/nvme1n1

Finally, update GRUB:

grub-update

Depending on your system and the specific issue, you might need to take additional steps to get the system working. This guide serves as a baseline for troubleshooting when systems won't boot.

If the system is still not able to boot, then try to investigate the boot partition

a few note worthy things that could be an issue:
1) fstab no longer has the correct disk for boot
2) grub is not installed on the disk install with

grub-install /dev/disk0n1