It's not 100% clear to me how your system is installed. My suspicion is that you've got a BIOS/legacy-mode Ubuntu install and an EFI-mode Windows install. It could be that something else is going on, though. Running the Boot Info Script and posting a link to the
RESULTS.txt file that it produces will give us details about how everything's set up, which would help in proceeding without guesswork.
That said, and proceeding on my initial guess, you could try this:
- Reconfigure your boot order to boot Linux.
- In Linux, type
sudo parted -l and look for a FAT partition with a "boot" flag set. If I'm right, this is your EFI System Partition (ESP). If you're not certain you've found an ESP, stop here and post that Boot Info Script output.
- Mount your ESP at
/boot/efi, as in
sudo mount /dev/sda1 /boot/efi if your ESP is
/dev/sda1. (You may need to create this mount point first, as in
sudo mkdir /boot/efi.)
- Back up your ESP. Copying its files to a convenient location or creating a tarball of the
/boot/efi directory should do the trick.
- Download the Debian-package version of my rEFInd boot manager.
- Install the rEFInd package. Double-clicking it in a file manager may do the trick, or you could type
sudo dpkg -i refind_0.6.6-1_amd64.deb (adjusting the filename as necessary if I release a new version before you read this).
With any luck, when you reboot you should see rEFInd, which should enable you to boot either Windows or Ubuntu without the help of GRUB. In a worst-case scenario, the computer won't boot any longer, which is why I had you make a backup of your ESP -- you should be able to use an emergency disc to restore everything to get it booting again.