No option to boot Ubuntu after Windows 10 update in dual boot [duplicate]

I had Ubuntu 16 and Windows 10 installed and coexisting peacefully.

On boot, the past and wise me had configured the computer so that it would give me a choice on whether I would like to boot Ubuntu or Windows, on a timer that would start Ubuntu by default if 30 seconds without input passed.

But the last Windows 10 update has disabled the possibility of booting Ubuntu altogether.

What I have tried:

• I entered the power options on Windows 10 and disabled fast boot
• I have shutdown the computer while pressing shift, then pressed F2 on boot to access the BIOS menu. There I have checked that indeed my first boot option is Ubuntu, and I have also tried disabling a fast boot option there was there in the BIOS menu.

I do not know what else to try.

How can I return the boot configuration to its behaviour pre update?

UPDATE:

• I have tried running bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi from the console with admin rights. When I load, now I see a scary screen saying

  Secure Boot Violation. Invalid signature detected. Check secure Boot Policy in setup

• I have also tried booting from a live USB. Got into BIOS menu using F2 on boot, then when I launch the boot override... I get the same scary message as above.

You can probably boot to an Ubuntu live disk/USB and run Boot Repair to restore your GRUB bootloader, thus, hopefully fixing your Windows or Ubuntu choice on boot.

Have you checked your partitions? The Windows 10 Creators update is known to delete linux partitions.

In most cases, restoring the partition is sufficient to restore boot unless you have made other changes.

testdisk is a good program for recovering partitions and is available from the repository.