If you are dual booting Windows with Linux, it is a common problem for many users that after upgrading Windows to a new version it will reset the bootloader to Windows Boot Manager which will cause Grub not to load anymore and therefore not possible to choose another OS to boot from.

Here we have a fix for you!

1- In Windows, go to start menu:

2- Open Windows command line as administrator:

3- Enter this command:

• Ubuntu

  bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi
  

• Other distributions:

  bcdedit /set {bootmgr} path \EFI\"distro name"\grubx64.efi
  

  Change "distro name" with the codename of your Linux distro, for exemple:

Debian = debian
Arch = arch
Fedora = fedora or use bcdedit /set {bootmgr} path \EFI\fedora\shim.efi

Didn't work?

Reset the registry key to default by doing so:

bcdedit /deletevalue {bootmgr} path \EFI\ubuntu\grubx64.efi

Even if the above command for deleting value didn't work, set the key back to boot into Windows normally.

bcdedit /set {bootmgr} path \EFI\Microsoft\Boot\bootmgfw.efi