How to fix grub error file not found

Use Boot Repair to fix your bootloader.

1. Boot Ubuntu from a LiveCD or Live USB
2. Connect to the internet
3. Open a terminal, and add the Boot Repair PPA
   sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update
4. Install Boot Repair
   sudo apt-get install -y boot-repair && boot-repair