Hello!
Sounds like you need Boot Repair. This will fix the GRUB bootloader for you by reinstalling it. More info here:
https://help.ubuntu.com/community/Boot-RepairIt is available as a stand-alone Live CD, but personally, I've never had much success with using it that way. Instead, I do this:
While keeping in mind that these changes won't be saved to a Live CD or USB, on an LL Live CD/Live USB, I add the Boot Repair PPA:
sudo add-apt-repository ppa:yannubuntu/boot-repair
,
then I enter:
sudo apt-get update && sudo apt-get install -y boot-repair && boot-repair
,
and follow the prompts from there.
If you want to have this available all the time, you can create a Live USB with persistence, but I just installed a small LL system on a 16GB SD card and added to it boot-repair and another goodie, ClamAV, to check Windows PCs for possible virus issues...
73 DE N4RPS
Rob