After many tries i got a solution, hope it helps other people... Similar to duplicated post suggested but bit different as i have an partition for EFI:
What worked for me:
0) If not installed ( i have 64 bits system):
sudo apt-get install grub-efi-amd64
( this solves problems like Permission denied error /usr/lib/grub/i386-pc/modinfo.sh doesnt exist.Pleease specify --target or --directory).
1) Install gksudo:
sudo apt-get install gksu
2) It didnt work for me, but you dont loose anything to try it:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install boot-repair
Execute boot-repair and use Recomended Repair.
If it doesnt work:
Observation: ( if live cd mounts any of this partitions, unmount them before mounting again in these folders):
4 ) As my / partition is /dev/sda8
sudo mount /dev/sda8 /mnt
5) This step Only if you have a separate boot partition ( my case /dev/sda6):
sudo mount /dev/sda6 /mnt/boot
Only if you have EFI in another partition also: mount the EFI partition . This solves errors like seems no efi partition
sudo mount /dev/sda2 /mnt/boot/EFI
6) Mount the critical virtual filesystems:
sudo mount --bind /dev /mnt/dev
sudo mount --bind /dev/pts /mnt/dev/pts
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys
7) Chroot into your normal system device:
sudo chroot /mnt
8) If there is no /boot/grub/grub.cfg or it's not correct, create one using
update-grub
9) Reinstall GRUB 2:
Substitute the correct device - sda, sdb, etc. Do not specify a partition number. In my case /dev/sda
grub-install /dev/sdX
10) Verify the install (use the correct device, for example sda. Do not specify a partition):In my case /dev/sda
grub-install --recheck /dev/sdX
11) Exit chroot: CTRL-D on keyboard exit
reboot
Optional: install grub-customizer in order to alter names and position of the entries