Playing around with GRUB is a nightmare for the first few times, and often GRUB reinstallations will actually be mandatory when installing multiple operating systems on the same drive, or multiple drives, or after reinstalling windows or vice versa.
If you can’t boot into your OS, you’ll need a linux live CD of any kind (Debian or Ubuntu) or a rescue disc. The most natural way to reinstall GRUB is to boot into your main OS via a live CD and run the grub-install command. I prefer not to do this as my live CD is around 4GB and can take 5-20 minutes to create the USB. And if you don’t have your OS live CD laying around, you’ll need another PC to either download it and create the bootable USB (60 minute exercise at least), or you can use a smaller, lighter, recovery OS on a bootable USB.
The best rescue method is by ‘burning’ Super Grub2 Disk to a USB or CD/DVD. You’ll need to download the Super Grub2 Disk ISO from here: https://www.supergrubdisk.org/super-grub2-disk/
It’s a tiny 20MB ISO that you can use Etcher to create a Super Grub2 Disk bootable USB in about 30 seconds. Any version will work. This will get you back into your PC. Once you’re inside you can run the grub-install command. On Debian this is the only command that worked for me completely:
Open gparted or gnome-disks and find the /dev/sdX of the drive you want to install grub to. Your main partition will be either /dev/sda or /dev/sdb or /dev/sdc or /dev/sd0. It should show that it’s mounted at / which is your root directory of the OS you’re in, you’re main OS.
grub-install --force --debug --boot-directory=/boot /dev/sdabc0
Replace sdabc0 above with whatever your drive is.
Now reboot without the Super Grub2 Disk USB and see if your distro goes into GRUB. This is a good time to keep the Super Grub2 Disk in a safe place as it is a very powerful tool for booting into all sorts of drives or places, even ISOs with loopback setup.
How to use gparted live when your GRUB isn’t working with Super Grub2 Disk and when you only have one USB.
Now, here’s a challenge: rescue your linux computer with one USB.
Get Super Grub2 Disk as described above and boot into your normal OS via the above method. After you’re in your normal OS, download Rescatux but don’t burn it to a USB.
Instead, as root, open your boot folder and make a folder called boot-isos and put Rescatux in that folder.
Download the latest Rescatux ISO from here and put it in that folder.
Now when you boot using Super Grub2 Disk, you’ll see a bootable ISO entry with the Rescatux ISO which you can boot into! This is because Rescatux is setup for loopback mount meaning you can boot into it on a hard disk instead of a USB. Booting a live distro from live USB. Rescatux has gparted and you can make changes to your partitions without being mounted but Rescatux does not have grub-install. This helped me when I had one single USB but I wanted to use both “gparted live” and Super Grub2 Disk at the same time without losing my Super Grub2 Disk USB.