I might try repairing the grub install. Not sure if just a reinstall of the boot loader to the MBR or a full grub uninstall & reinstall is required. Lets try simple first.

Boot Repair -Also handles LVM, GPT and separate /boot.:
https://help.ubuntu.com/community/Boot-Repair
You can repair many boot issues with this or 'Create BootInfo' report (Other Options) & post the link it creates, so we can see your exact configuration.

If that does not work, run the Bootinfo report and post the link that creates.