If Grub fails to boot your operating system, you can take steps to find and boot your system then reinstall grub to fix the problem. The following example shows you how to use commands in the emergency grub command line to find your operating system and boot it. Lines that start with the comment character ‘//’ is a description of what the command is doing.
// use the 'ls' command to find all partitions that grub can find
grub rescue> ls
(hd0,gpt1) (hd0,gpt2) etc
// use 'ls' to see what filesystem each partition uses
grub rescure> ls (hd0,gpt2)
Filesystem is ext4/btrfs/etc
// use command to see what the filesystem contains
grub rescure> ls (hd0,gpt2)/etc/fstab
// now that you've found your operating system, boot it
grub rescure> set root=(hd0,gpt2)
grub rescure> set prefix=(hd0,gpt2)/boot/grub (or grub2)
grub rescure> insmod normal
grub rescure> normal
// Once you're back in the system, fix grub
# sudo update-grub
or sudo grub-mkconfig –o /boot/grub/grub.cfg
or sudo grub2-mkconfig –o /boot/grub2/grub.cfg
or sudo grub2-mkconfig –o /boot/efi/EFI/fedora/grub.cfg
# grub-install /dev/nvme0n1 (or /dev/sda)
or sudo grub2-install /dev/nvme0n1 (or /dev/sda)Make sure that you know where your grub.cfg is located and the location to install grub (/dev/nvme0n1, /dev/sda, etc) and modify the above appropriately.
LinuxMunky 