Installation of Windows 10 & Ubuntu 18.04, Ubuntu 18.04 & Windows 10 [duplicate]
You have to recreate your grub.
- boot from live-CD
-
mount your filesystems from your original installation e. g.
sudo mount /dev/sda<#> /mnt
-
chroot
sudo chroot /mnt
-
recreate grub
sudo grub-install --recheck /dev/sda
If you are using LVM you have to mount /dev/mapper/ /mnt
https://unix.stackexchange.com/questions/226169/chroot-copy-or-mount-bind