Instead of having the Grub menu appear to make a selection of which OS, I get the message:
Failed to open\EFI\ubuntu\grubx64.efi - Not found
Failed to load image \EFI\ubuntu\grubx64.efi: Not found
start_image() returned Not Found, falling back to default
The computer then goes into Windows.
I am a newbie, so please be explicit about any solution. Ubuntu 18.04.