Assuming your grub is installed correctly you need to re-generate its configuration.
In a terminal, the steps to do this are as follows. You need to mount the partition in a chroot. This can be done quickly when using a Linux Mint live DVD.
Escalate your shell to root because you will need root privileges to do this process.
sudo su - |
List your disks to find your install. Look for this.
┌──(john㉿DESKTOP-PF01IEE)-[~] └─$ lsblk | grep / sdc 8:32 0 256G 0 disk / |
The disk device associated with the / partition is what we are after.
Mount your disk to /mnt
mount /dev/sdc /mnt |
Mount the special filesystems required to chroot into your installation and perform repairs.
mount --rbind /dev /mnt/dev mount --rbind /sys /mnt/sys mount -t proc proc /mnt/proc |
chroot into the installation.
chroot /mnt /bin/bash -l |
Check that /boot and/or EFI partitions have been mounted correctly
mount | grep boot |
Re-generate your grub config.
grub-mkconfig -o /boot/grub/grub.cfg |
Reboot and fingers crossed that grub will work this time.