sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt update
sudo apt install -y boot-repair
boot-repair

sudo mount /dev/sda3 /mnt
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys
sudo chroot /mnt

apt remove grub-efi grub-efi-amd64
grub-efi-amd63-signed shim
apt install grub-pc
grub-install /dev/sda
update-grub
exit
sudo reboot